How do I make it so a player leaves their units are given to the computer?

to access player 12 you will need scmdraft or starforge

this works even for whole forces.
And should be able to use x-tra editor too if you use P12 mode.

errr, there is a problem with that trigger, it is saying if p12 commands 1 unit at anytime anywhere, than every single unit everywhere will be given to p12... and I highly suggest using more specific terms than just "any unit", because that includes EVERYTHING; buildings, beacons, powerups, map revealers, etc.

To access P9-P12 triggers for Xtra, you need to import some triggers.

the attachments are the triggers for each P9-P12. It will not specifically say Player 12, but be a blank space. Leave this alone as you can't get it back if you change it. The best way to use these triggers is to copy the entire listing onto a new trigger, delete what you don't want, and add what you do want. In your case it would be, P12 brings any unit to anywhere, Remove/Kill all any unit for P12.

And as Akar said...P12 actually controls more units than just men/buildings. Minerals are also given to P12, so you should remove/kill specific units or just men/buildings.

that is what a lot of maps do, like defense maps for example...

yeah, and if it is a defense type map where you need those buildings/men saved, just give any men/buildings from P12, back to the players that are still there. The current risk map does this when someone leaves so that there is not free kills.