I haven't caught any on YouTube yet (knock on wood), but in general you definitely want to just ignore them. Dislikes/negative comments still help your videos rank higher and if they're really dedicated they're probably watching your videos 3 times to find something to hate you for, which helps cancel out the audience retention hit you take from spammers who pause your videos a couple seconds in. If they get really aggressive/threatening, block them, but yeah. Just ignore them and consider the fact that this is just one more step on the way to officially saying you've Made It.