I don't think the decision to come off or not should be made by the team whose players are being abused. Once it gets to the third stage of UEFA's protocol the referee should automatically make the decision to take the teams off and abandon the game. The team whose fans are offending should then forfeit the game.
There is a problem with racism in eastern Europe generally. The monkey noises and chants are very reminiscent of what it was like in English grounds in the early 1980's. The problem hasn't totally gone but it is a world away from what it was like back then. I remember going to a match where Alex Williams the Manchester City keeper was pelted with bananas.
UEFA has been too soft on this for too long and are only belatedly getting tougher. I agree to some extent with Llani's point that there is a certain hypocrisy in England - some of our fans regularly disgrace themselves as we know. It has long been a national embarrassment. But it's not a reason not to act on racist abuse. Countries can overtime control what goes on in their stadia by issuing banning orders. If they don't even acknowledge the problem, which appears to be the case with Bulgaria then eventually they shouldn't be allowed to participate in international tournaments.