Modify opac-modrequest.pl to have it check to see if item has arrived for collection, and either suppress Modify option to prevent users cancelling the reservation (hold) when item is ready to collect, or warn the user the reservation (hold) cannot be cancelled as it is now too late.
On master and I think a few versions back a waiting hold can't be suspended or cancelled from the OPAC. As this was filed against 16.05 I'll set to RESOLVED FIXED.