People are always kissing me, and more often than not they've got garlic breath, which really gets my mood down. Is there any way of preventing people from kissing me?
You probably know this already, but those players are attempting to complete the early quest Spread the Garlic Love. There's currently no way to block these types of 'social' player to player interactions globally, and given the nature of the game I doubt there will be one any time soon.
Instead, since this is a relatively early quest, you might want to simply avoid areas where new players start, or move to less crowded areas. Other than changing your username to "Stop kissing me" I doubt there will be a better way to prevent this, so I suppose you should just take this as part of the game's humor and go along with it. The mood loss is fairly small anyway, and you won't suffer any penalty until your mood drops below the half way point.