Campaigns potentially have two separate instances defined for them in the class PegaMKT-Work.  One starts with P-, which is the campaign information entered by the user, while the second starts with PR-, which holds the information for a run that is initiated.  I've had luck just manually deleting both of these instances using Obj-Delete in a temp activity without any observable regressions.  Not sure if this is the correct method, but it has worked for me. 

@IvonneR8

Please review the documentation here for work object archive / purge : https://community.pega.com/sites/default/files/help_v718/procomhelpmain.htm#tools/purgearchive/aboutpurgearchivewizard.htm

I would highly discourage you from directly deleting instance like suggested in the above thread, given the dependencies in the index and other tables.