I think what needs to be done, if not a script-enforced lag cap, is that players need to be sent a server PM warning them that they are at risk of being banned if they continue to spar with a ping over Xms. Then, staff can determine that they've been adequately warned by the system and know it's fair and right to ban them if it continues.
I bet 90% of the people who are banned for allegedly lagging too much would never be banned if they knew it was too much, simply because "too much" is very hard for your average GP to adequately determine and it's an inconsistent opinion across multiple GPs... It's very hard to follow a rule when you aren't even sure when you're breaking it.