The questions raised by TheLittleIKnow are good ones that really should be addressed by a halachic authority; however, I will hazard a few guesses.
Does despising someone mean…
…attacking them violently?
Probably not.
…feeling nausea in their presence?
Probably. If someone were to smack your parents in the face, would you not feel nausea in their presence?
…ignoring them, not conversing with them, refusing to work in their proximity?
Conversing with them should be kept to a minimum.
Yiddishekup: You make a very valid point; thank you for bringing that up. If someone struggles in a certain area, wants to do the right thing and tries hard, but sometimes messes up, such a person should not be called a rasha. There’s no mitzva to hate such a person.
However, the vast majority of people being discusses here do not fall into that category.