Why can't the AP be secured?
If it's someone whose supposed to be using the AP that is responsible for the questionable behaviour as identified by the ISP, then no wireless security measure is going to help anyhow.
An acceptable use policy may be some cover from any comeback on yourselves from it's abuse (I'm not a lawyer, so don't take that advice as absolute).
If it's someone that's not supposed to be using it, the problem with any disassociation method is that it will be MAC based as far as I'm aware. Whats to stop them from changing their MAC address and regaining access?
Anyhow, in addition to the advice already given, Aireplay from the Aircrack suite can also achieve this.
Security + | OSWP | eCPPT (Silver & Gold) | CSTA