I've been here for about a year and a half, so I have about 20 people blocked over that time. Some of them I really can't remember why I blocked, I've seen them online and they seem okay, but know that back then, it was for one of these reasons:
1. They were personally rude to me. Disagreeing with me means nothing. If they took it another step further and told me to go get cancer and die, or if they made an ignorant remark about what they think I *must be like* then I don't have time for the baby shit here. I realize it's the internet, but in life I don't tolerate this shit, and the only way to stop it online is to block.
2. They were harassing. If I received private messages and asked them to cut it out, then the only way to knock it off was to block.
3. They showed offensive behaviour to others. I might have been spared from abuse from someone, but if I notice they take things way too far with someone else - and it would have had to been pretty bad, then I'll block to save myself any future problems from them.
I won't block just because we've disagreed. I enjoy a lively debate or discussion. In fact, some discussions have amicably ended with an agree-to-disagree. I've also noticed that the person is decent to me in other topics and nothing ever got to the point where things were said with the intent to annoy or hurt the other person. If we can keep it clean, then I even consider some of these people GaG-friends because they know the difference as I do when it comes to discussing in a healthy/friendly manner as opposed to blatantly being weird and going out of one's way to be discourteous.
I will and have unblocked after receiving an apology. This is from someone going through another user or mod to make the effort to apologize, or if they say something within a topic that I can see expressing that they hope I unblock them.
There is a user here who wisely once said, "I will block when they interfere with my enjoyment of the site." This is how I view things too. I don't come here to get wound up like a top and get headaches and feel stress. I come here to relax and help people and hope for the same thing back. If someone is using the time here to be angry and pulling me in to their obvious negativity, then they can go look for other targets. It doesn't mean the classic "Oh I just won" or, "I guess I'm too smart for you". No, it's that I don't waste my time on bullshit offline and especially don't online.