A previous smoker’s bathroom…

We just purchased a home and didn’t notice when before we started living here. One bathroom in particular smells like cigarettes periodically.

Doing sniff tests and looking at other clues we have, I’m 99% sure someone at some point, used this as a smoking bathroom, and probably had the vent fan on to push the smell out. This seems to have let the ductwork get saturated with residue, causing it to leak back down into that room. At least that’s what my sniff tests and repainting already tell me.

Short of removing and replacing the ductwork (very intrusive with the way it runs out of the house), are there any tricks we can do? I’m wondering about renting an ozone generator, closing the bathroom up with the vent fan on and letting it go for 12 hours or so? Would that do anything or are we pretty much SOL?