Most deletion tasks in Phabricator are deliberately unavailable from the web interface. You probably will need to use the
phabricator/bin/remove CLI utility.
remove - remove objects
remove command [options]
Administrative tool for destroying objects permanently.
destroy [options] object ...
Permanently destroy objects.
Show this help, or workflow help for command.
Show a log of permanently destroyed objects.
I must admit I do not know how to get the PHID of a Task form.