A320 Captain reported a fumes event during deplaning at destination. Flight attendants refused to fly with the persistent odor after aircraft had been cleaned. Captain and First officer agreed to ferry the aircraft to maintenance base for repairs.

Narrative

After landing in ZZZZ; the aircraft started to accumulate a very nasty smell. We had just landed; turned the APU on; and proceeded to park at Spot X at ZZZZ. It was raining heavily on and off since a rain front had just passed over the field. We had only one operating Pack at the time; the number one Pack was operating as the number two was on MEL. The customers starting deplaning when the number four Inflight came to the cockpit; wearing a mask and blanket over her face stating she had to leave the aircraft because an odor in the aircraft was becoming overpowering. The cleaners then came onboard and every one of them complained about the dank; wet odor in the front of the airplane and did not want to clean the aircraft. Their supervision overruled the cleaners' opinions; and they quickly cleaned the aircraft. After the cleaners left; local AO wanted to board the aircraft however; the number four Inflight stated she would not fly with that odor on the plane. I asked all the Inflight if they agreed with the odor assessment and not boarding. They all stated that they felt very unsafe in that environment. I agreed with them; stopped the boarding process then my FO (First Officer) and I proceeded to run the odor isolation checklist to figure out the issue. After running the odor isolation checklist; we determined the odor to be emanating from the remaining number one Pack. The predominate location of the smell was between rows 4 and 5 and the smell; in my description; was like a wet rag laced with some chemical odor that was warm and burning. We shut off the Pack; opened all doors to dissipate the odors and the smell in the aircraft finally cleared out.Local maintenance came out and was directed by Company Maintenance to put the remaining Pack on MEL. My FO and I volunteered to ferry the aircraft back to ZZZ for repairs.