In RPM processes it is possible to do a mass deletion of forms using the currently active view. This is useful if you want to remove older forms, or forms from an entity that is no longer active. 

Top Menu >> Processes >> Select process

Make a view to limit the active the forms displayed using a custom view. Once the view has been created click on the More menu at the top. 

In the More menu select Trash

You will now receive a warning indicating how many forms are to be deleted. Click Trash to move the forms to the trash bin.  

Once the forms are trashed they will be permanently deleted after seven days. A form that has been trashed can be recovered by the user whom deleted the form, or an RPM system manager during that seven day period.  

Further reading