1. do the pun-pun trick, pazuzu, divine minion, either way it doesn't matter,
2. pun-pun has his familiar(or psi-crystal, etc...) give him an EX ability that takes away all of his ex, su, and sp abilities and makes his ability scores str 0, dex 0, con 1, int 0, wis, 0, cha 0 : add change into a sandwich or a banana, a clump of raisins it doesn't matter
3. enjoy your hellish existence, helpless in five different ways, unable to move and having nightmares
pun-pun is also the weakest character ever, if he wanted to be/ wanted to commit suicide
the only thing to add, i guess, would be a permanent version of every spell that could affect him in a negative way(without killing him) and every adverse condition, blinded, dazzled, deafened, confused, cowering, dazed, entangled, exhausted, fascinated, panicked, flat-footed, fatigued, pinned, sickened, petrified, paralyzed, turned, nauseated, knocked down, invisible, incorporeal -[yes some redundancy but pun-pun can do that]
(is there incorporeal mold to worry about?)
edit:add some kind of fast healing/regeneration/ re-appear if you die- lich style , so he CAN'T EVEN DIE!! - permanently