Gosh, this is an interesting topic, trying to write it down has made me realise how complex it is!
All of these situations have the same general rules, game disruption. The important thing to note is that I always warn people first as to the problem, then I warn them that it will result in blacklist well before I do it. It's not actually very easy to get onto my blacklist, you have to really want to be on it. Although I have found these days I tend to Blacklist people quicker, once I see a pattern of behaviour in a player, I generally know where it's going and nip it in the bud. Saves both of us a lot more time... I would say the easiest way though would be by request. Some people don't like the sort of games I host, and many other reasons, so I blacklist them if asked.
To get unblacklisted is very difficult, mostly because if you didn't listen to the warning before, then I'm not sure why you would in the future. Tojam managed it after a very long time of apology, explanation and because of overlapping game preference. (He learnt from his mistake and we happen to play a lot of similar games, basicaly.)
Ok, the reasons:
Declare someone as PE without first checking the facts of the situation (especially if you're not a player involved and double-especially if you're the Host) as it generally completely ruins games. Players might read the PE claim, attack the player, then they become PE because it was a mistake, etc etc. It often escalates very quickly and I get sick of games which are 90% people arguing over who is PE. Always a waste of time, and I tend to be less tolerant of this these days, especially in big games that take a long time to organise. Stop wasting peoples time because you can't get over a simple attack on something, it's a war game, grow up, sort it out, talk it out, negotiate, whatever...
Playing in a "game-manship" manner if it's against the spirit of the game in question. This doesn't normally apply in strat or FFAs, but for example in a Diplo, attacking a player who might be booting. Yes, technically the attacks will hit neutrals, but only if they boot, and the attacks were still set up on a player (Think: Could I attack them if we were Diploed? If the answer is no, don't attack!)
Continuing to host rigged games after being warned.
Suiciding in a FFA. This normally only applies to FFAs, as the aim of such games is to win, attacking a player to the point where both players lose is mind-bogglingly stupid! This rule is more lax in Diplos if you're doing some kind of role-play (it happens in historic situations)
Being a relentless troll. Normally I'm pretty lax when it comes to players being rude or annoying verbaly, but if it's constant and direct, after a good few months and a warning I generally Blacklist them (Best and Karl are good examples of this)
Deliberatly becoming PE in a Diplo or teaming up with a PE. There are exceptions however, if a player is backed into a corner, or has no chance but to get the jump, that's understandable. Even the Japanese did that in the attack on Pearl Harbour. If a player apologises/didn't understand/corrects, then that's also fine, it's better to just get the game rolling again.
Dragging out a game. This is particularly applicable in Real-Time games, but often in a Multi-day, especially in a Diplo where everyone agrees on a winner, but then a player decides to drag the game out for no clear reason other than being bitter or whatever. Just get over it, and move on to the next game, there's always another game waiting...
Playing for self-gratification. This is a bit of a catch all, but the main reason I personally play Warlight is for the social aspects of it, the interactions with players, the banter, the discussions over tactics and gameplay and settings. If you're disrupting other players fun, or playing like an ass, or just generally being a twat, I'll blacklist you for it. Again, I always warn players, but generally players like this will seldom care and respond with "so profanity what?" and... yeah, easy decision on that one. Mostly this is because I invite a lot of my friends to my games, and I want a chill atmosphere. Players like this will have no qualm in eliminating a player just because they "feel" like it, so in the end I'd rather keep them out my games. Which is 95% the reason I use my blacklist.
By request. If you ask me to Blacklist you then I will, it's the only courteous thing to do, some players don't want play the kind of games I host (which tends to be numerous) and so don't want to see them on their list, and many other reasons.
I'm happy for others to Blacklist me too, if they don't enjoy playing with me then I would rather they just didn't. Warlight is big enough that we can find other games to play. Kongregate doesn't have a blacklist system, which is a real pain. I think it's a great tool for the community to use to improve individual experiences in the games we all play.
TL;DR: Just don't read it, lol...