Cheers begin to cross the line when they go from support foryour own team to negative, taunting comments directed toward the other team.
There is nothing in writing that says "this cheer or that cheer" is illegal. Any unsportsmanlike comments are to be judged by the umpire on their own merit, and it is strictly a judgement call. So the judgment can vary from umpire-to-umpire.
Different organizations or different leagues might have their own rules on how to handle this. That can lead to even more variation in the enforcement of this rule.
As an umpire, generally you have no control over cheers coming from the stands. But you do have a say in what happens on the field. If a situation like this is getting out of hand, the umpire needs to deal with it quickly before any bad feelings can escalate. If one team is being taunted, there is a very real possibility that the taunting can lead to retaliation. Let it get out of hand and things can ramp-up to batters being hit by pitches, runners crashing into fielders or other dangerous, unsportmanlike activity.
In the immortal words of Barney Fife: "Nip it in the bud, Andy, nip it in the bud!".
If this gets out of control in one of my games, I'm going to have a talk with the coach of the offending team. If the taunts continue, I'm probably going to issue an official warning at that point. I will give the coach a little bit of leeway here, as the rule is not crystal clear what comments are allowed and what are not. Maybe we just have a misunderstanding as to what is acceptable, since the rule falls into a somewhat grey area. I'm going to give the coach a fair chance to get his team under control so the game can continue without interuption.
By now, the offending team and coach are on thin ice. They have been warned and any continued abuse will be subject to ejection or possible forfeit (as a last resort).
Beyond the written rules, this situation is handled with good game management techniques applied by the umpire to keep the game under control.