If you are already running a strfry relay for this then one solution is to add the NIP-40 "expiration" tag to events your bridge produces, strfry will respect those. Another option is to run a simple script every x minutes that queries all events before a certain timestamp and deletes all of them. You can use the "./strfry scan" and "./strfry delete" commands. Adding the "-" tag is a good idea too just to prevent unwanted spreading. After this you can safely claim that messages are not persisted (of course if someone wants to maliciously persist them they can do that, but one can also do that with OwnCast server messages I imagine).