If someone insults you or you get a spammer answering your question or Notebook entry then I would block them.

Adding to Grant's answer, I might point out that at least in American English, if you "pick up" someone online, it implies you met that person in real life, and probably had sex with him. If you did not mean to imply this, perhaps it would be safer to write "when should I block someone I friended online?"

If you're picking people up on the Internet, then you deserve the trouble you get into. Just be more careful.

First, it's "good riddance". Radiance is a quality of light. Second, that's really up to you. My personal opinion would be, if the person starts trying to overtly sexually solicit you or stalk you, or they're otherwise being obnoxious and rude, then you should block them. But I'd also suggest not trying to pick people up online. I'd also suggest not picking people up. People are icky. I prefer video games. Pick up some video games instead.