What it means for a virtue to regulate an appetite has nothing to do with diminishing appetite as such. It has to do with ordering an appetite toward what is truly good for us. In some cases we may have an excessive appetite, but in other cases a deficient, appetite. Training in virtue, therefore, for some individual, may be just as much a matter of stoking desire as quelling it.
One thing often misunderstood about excess and deficiency as these pertain to our desires, choices, and actions, is that there is no absolute cap on how much something may be desired, or how intensely it may be pursued in action. The goal of the virtuous life is a right ordering of loves, not a right amount or degree of them. We can see this by asking ourselves whether it is possible to enjoy a favorite food too much. Surely it is possible to have too much of a favorite food; but it is not possible to enjoy a favorite food too much. The perfectly temperate person might take just the right amount, eat it at just the right time, but enjoy that meal with a greater intensity than ten gourmands.
Eating moderately is, of course, not the only condition one must meet in order to enjoy the pleasures of food in an all-things-considered good way. If you are savoring every morsel of your moderately portioned filet mignon while the diner next to you is having a heart attack, you are eating viciously. This is not to say you aren't truly moderate in your eating habits if you are unjust toward others. No, you really are moderate. The problem with your moderate but intensely pleasurable steak-eating is not the moderateness of the act, or the intensity of the pleasure, but rather the context in which the pleasure is had: next to a dying man, at the expense of failing to help him.
The moral saint--the perfectly virtuous person whose life achieves this right ordering between desires and actions, delighting in pleasures, disdainful of unnecessary pain, but also sensitive to the needs of others and eager to act for their good and for the common good--such a person has the moral green light to love everything he loves fervently, passionately, taking delight in delightful things to a degree which must appear foolish or hedonistic to outsiders. Let's say, somewhat simplistically, for brevity's sake, that the moral saint will love what is good in every good thing, and will love every good thing in proportion to its goodness. Thus, if one thing is better than another (e.g., a human being is better than a single charcoal briquette) the saint will love the human being as much more than the briquette as the human is better than the briquette. But this proportion tells us nothing at all about the degree of intensity of the love the saint has for the coal, or the human.
We may say similar things for hatred and anger. How much is it permissible to hate what is evil? That's not a good question. What is worthy of hatred, and what makes something more worthy of hatred than another, are very good questions. The sort of hatred of adultery which we associate, almost totally pejoratively, with seventeenth century Puritanism is bound to seem irrational and wrong if we are thinking of marriage "back then" as intrinsically oppressive, or something like that. Were we instead to think of it more like it really is: the breaking of a freely given promise to be the one person in the world on whom another can totally rely and build his or her life around--well then, adultery starts looking pretty bad. But if your hatred of spousal abuse (which is often what makes adultery an attractive option) is not proportionately intense, then there is something wrong with your hatred of adultery--again, not its intensity, but its proportion to your other hates.
We must also say that whatever a feeling of hate is, if it is good, it is the affective mode which love takes when something loved, whatever it is, is unduly harmed or in danger.
Love, eros and agape alike, has been described as a kind of fire. In pleasure or pain, affection or hatred, I like to think of virtue as the best fuel for the fires of love.
(On a slightly more scholarly note:
I think many of those whose ethical language is roughly the language of virtue and Aristotle's ethical vision will chafe a bit at the emphasis I'm placing here on feelings. That subjective element, how one feels about doing good, is not of central concern in what Aristotle has to say about what the flourishing human life looks like. True, the virtuous person delights in doing good, and spurns what is evil, but, it is often emphasized, these feelings are a by-product and not the goal of virtuous action.
Now I believe all this, but at the same time I also think the Aristotelian tradition in ethics does not get right, or has often failed to emphasize, the importance of intense feelings to a good life. The kernel of wisdom in the German and English Romantics is that our capacity for certain affective states includes the capacity for extremes of these states, that being the subject of these extremes often gives rise to a second order affective state of feeling "alive," and that the experiences which gives rise to these first and second order states are often taken to be especially "meaningful." That there can be falsity and even superficiality in these extremes and higher order states to which they give rise, I don't deny. At the same time I also affirm that these experiences can be legitimate and salutary in the sense that they remind us that, boring and vain as life often feels, it all really does matter: something Big is going on and you are wrapped up in it somehow.
Appreciating this Romantic insight, I suppose one way to say what I'm after in ethics is an understanding of virtue as that state of the soul in which these Romantic extremes may be felt and relished to their Dionysian full, and be good for the one who feels them, and be good for those in the vicinity of those raging fires of love.)