Can all users delete tasks (process instances)?
Only administrators (Admin role) and the Owner of a given Organization have such permission.
You can read more about roles and permissions here
And about viewing and changing permissions here
However, there is one situation in which a “regular” user can delete a process instance – he has this right if he created it himself.
He can use this permission before he sends the task to the next person(s) and when the task comes back to him for the first step. This is a necessary functionality, because the idea is not to clutter the system with instances of processes that will not be processed anyway when making a mistake noticed at the start of the process or indicated by subsequent participants.