This has been asked before, but answers often center around "don't use the lock" or put something in the slot. The door to my patio/pool has deadbolts and a lever lock. No one ever admits doing it, but somehow the button on the lever ends up in the locked position. We were once locked out in night clothes. Putting something in the slot would keep the door from simply fastening without locking. "Don't use it" has been our intent all along, but something goes wrong. A new lock, etc, is not likely to help. I don't want to cut a connecting rod and destroy the lock.
I've put tape over the turn-knob, but that solution is imperfect. Get a different doorknob set that omits that "feature", and throw this lockset in a gallon Ziploc for refitting at sale time.
Since lock quality is not a factor, a $10 passage set should suffice.
Locksets that use a turn-button to prevent operation from the outside usually have a drive post that interfaces with the turn-button and extends to the latch; to effect the locking function. If you disassemble the lockset you may be able to remove and discard the post, effectively converting your lockset into a non-lockable "passage" lockset.
Super glue the button into the unlocked position.