Can't you use the WorkflowTasksManagement.AllowBreakLeases permission? I don't know if you need the .Modify permissions to use this. I'm not near a test environment at the moment so I can't certify that it does what you need but if you can, give it a go.
The lease is applied to the the task and is independent of the session. If you looked at this practically, if you wanted to lease a task and you closed your window accidentally or because you had to, you would still want the lease to be in effect. You wouldn't want someone else jumping into the ticket when your back was turned.
That said, if you don't use leases then just turn it off in the Master Settings > Workflow Settings, alternatively reduce the lease time from 15 minutes (default) to something more appropriate, say 5 minutes.