Let me use whatever browser I want to use.
If it makes your badly designed web page look like shit, I'll try another browser. Or, more likely, I'll pass on your badly designed web site entirely. But leave the choice to me ... that way, at least you'll have a chance of me trying another browser. Blocking me outright right from the git-go doesn't give you that second chance; I'll assume you're either hostile or an asshole and move on to greener pastures.