i am with Bec on this one..

Why close an affiliate account unless there is a very good reason for it... Just because they are not sending in buttloads of new traffic doesnt mean they will not start doing so again at a later time in the future..

If you close their account you might not get them back next time they start a new project that will send good traffic again.. They signed up and showed interest once so they might do so again if you leave their account open but closing it and you cant be sure they sign up again next time they have traffic that fits your program/sites..