The GPL FAQ addresses the issues being discussed here, actually.
1. If I use GPL, I can't stop someone from reselling my code:
http://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLRequireAvailabilityToPublic

2. If I use the GPL, that does NOT require me to support anything I release under it:
http://www.gnu.org/licenses/gpl-faq.html#SupportService

So, unless the support agreement promises support regardless of behavior, a developer can cancel support of someone who is reselling the developer's work against the developer's wishes. Though, personally, I'd explicitly state that as part of the support contract.