Can I ask why you think you shouldn't be able to transfer a ticket? I agree it should be limited but not stopped.
It just encourages touting and credit hunting, so everyone loses out. You might be able to think of the odd circumstance in which you think it might be reasonable to transfer a ticket in cases of genuine unforeseen circumstances but the reality is that whatever mechanism you put in place to allow that will be abused by touts and credit hunters.
The club already allow ST's to return tickets very late don't they?
Return the ticket to the club, get a refund, lose the credit and allow someone else to go who has got the credits or has paid their membership.
Based on what Phil Dutton has reportedly said this coming season is going to be a tout's paradise.