It shouldn't matter what browser you're using, that's the point of having web standards. I can understand not wanting to support certain users that use whatever

It shouldn't matter what browser you're using, that's the point of having web standards. I can understand not wanting to support certain users that use whatever browser you don't want to support, but a visible, non-intrusive and dismissible warning at the top should be enough. Otherwise, making radical decisions based on the User-Agent defeats the whole point of having web standards.