As I said 0 Exp IS the "fix" and it's the best you can do when the account is that messed up.
Some server teams I know will manually "re-create" an account which legally gets this messed up... especially if the problem is recurring with a particular toon. But they usually require authentic screenshots showing the stats and such that the character had.
If what you want to "fix" is the problem which causes you to over Exp in the first place; I can only suggest you re-do whatever changes you made to a standard server that allow your players to "over Exp", and get it right this time. XD
Having said that, I know that excessively "Uber" custom weapons and stuff can cause this kind of problem too... but I never see it as what I'm aiming for is always something as close to identical to the official servers as I can get. These issues occur because of private servers "Xblah Exp rate", "Xblah AP", "Max level 3000" etc etc. which the server was never designed to cope with.
You design modifications to make it work that way. So if it doesn't work right, it's YOF, and the only person who can fix it is you.
Of course... I know nothing about "Account Managers" either, as "official PT" never has those, and so I have no interest in them either. If that is the cause, I presume either you wrote the "account manager" = YOF or you know who did = "Talk to them".
The worst of all situations is that your server has massive Exp rate because... "I downloaded a server with modified Exp rate and I'm running it as my Private Server". That happens a lot (I suspect), but my advise is:- never do that. Then it's not your server and you are taking responsibility for someone elses modifications. If you are responsible for a private PT server, know every section of code that has been changed, and make the changes your self... base your changes on releases by others, but apply them to your own server don't just use someone elses.