The serverdelete command is there to remove a host from the system.
If you are unable to cancel a reservation from the web management console, it is unlikely that vcsadmin will do any different (they call the same API).
Normally when we see this kind of problem, it can be resolved by a VCS services restart.
We recommend that you check the engine manager to make sure there are no active deploys or teardowns, then shut down the surgient engine service, surgient service host, and the surgient control service, in that order. Then restart them in reverse order.