Kill them all?
On a more serious note, it's a long shot but making something updatable that gives a syndrome with the opposite effects of becoming a necromancer might work, but I'm not sure what's updatable without at least a fort reset.
Syndrome for necromancer:
[CE_DISPLAY_TILE:TILE:165:5:0:1:START:0]
[CE_DISPLAY_NAME:NAME:necromancer:necromancers:necromantic:START:0]
[CE_ADD_TAG:NOEXERT:NO_AGING:NO_EAT:NO_DRINK:NO_SLEEP:NO_PHYS_ATT_GAIN:NO_PHYS_ATT_RUST:START:0]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Animate corpse]
[CDI:INTERACTION:EXAMPLE RAISE]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:10]
[CDI:VERB:gesture:gestures:NA]
[CDI:TARGET_VERB:shudder and begin to move:shudders and begins to move]
[CDI:WAIT_PERIOD:10]
syndrome that *might* undo that
[CE_DISPLAY_NAME:NAME: : : :START:0]
[CE_REMOVE_TAG:NOEXERT:NO_AGING:NO_EAT:NO_DRINK:NO_SLEEP:NO_PHYS_ATT_GAIN:NO_PHYS_ATT_RUST:START:0]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Animate corpse]
[CDI:INTERACTION:EXAMPLE RAISE]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:0]
[CDI:VERB:gesture:gestures:NA]
[CDI:TARGET_VERB:shudder and begin to move:shudders and begins to move]
[CDI:WAIT_PERIOD:10]
But you're gonna hope someone better at interactions cleans that up, not sure how overwriting interactions works, and I don't think tile icons are resetable to default.