Mdk3 will suit your needs if scripted, or you can look into http://openwips-ng.org/index.html however it is immature at  this time.

You could try using Aircrack-ng to send deauthentication packets to kick the device offline....

http://www.aircrack-ng.org/doku.php?id=deauthentication

Disassociating clients is just a temporary measure that the client can easily work around. Better to just secure the AP itself to prevent these people from connecting. If the ISP expects you to secure it, tell them to give you admin access, or have them send someone with admin access to deal with it.

This is a basic rundown, we are using  temporary site, and there is a contract here that provides for wireless access. When we originally came to the site, the wifi was decent. We noticed significant issues as time went on, from users only being able to load 1-2 pages, ect. Well, the bosses called the company who did a survey. They stated it was users downloading from the living area while they were at the work site. Now I suggested that they ask the company to block the commonly used torrent ports and such, ultimately I am not in contact with the company myself, but my understanding is that the contract puts them in charge of the wired and wireless AP's, but we as the customer have some sort of support.

I would try secure the wireless to be honest as its the best way the other alternative is to use air crack and write de auth script

At which point it shouldn't be your problem, it should be the providers. Is the app and cable / dsl / whatever modem built in, or can you put a device between them?

if you can put something between them, I'd say go with the Untagled solution that jjwinter suggested.

Also have the person in charge let them know that what they are doing isn't authorized and there will be penalties for being caught.

I don't think I noticed anyone recommend this, but what's stopping you from unplugging the WAP in question and plugging your own in? Seems that'd solve all the problems.