This one really varies a lot from book to book.
Some like Pinnacle, Olympic, IIS, WSEX, Hollywood, CRIS, Grande and BetWWTS take on all comers up to their posted limits and give higher limits to certain players. At least I haven't heard of anyone getting booted or limited by any of these books.
Bodog and Bowmans use the dual line trick. This means that if you like dogs or otherwise pick off their soft lines, they will put you in a special category and give you different lines from what other players get.
SIA goes by amount won. Win about 10-15K and there's a security check first to see if they can find a reason to no-pay you, otherwise they'll just pay you and boot you.
PSB/V-Wager will limit you if you win about 5-10K betting close to their limits.
Wagerstreet is very quick to cut limits.
The others I can't really comment on but the typical practice is to review your betting patterns when you make a big withdrawal or 2 or 3 medium withdrawals. After that it's a subjective decision. They will try to decide whether your winning is because of luck or skill. If they think it's because of skill, they will likely take countermeasures.