I think it's brilliant for game balance that they continue to build.
There are a couple of opportunities that come from this Situation:
1) take over their generators ASAP to reduce the drill cap; the resigned/eliminated players factories will stop producing too many drillers
2) leave them and gift even more subs to the resigned outposts -- creating a "defensive wall" against you and other players. If they want to take the outposts, it will severely cost them.
I did option 2 in my last game; after eliminating a player, another stronger player wanted to steal the idle outposts. So I sent gifts that arrived before he landed to weaken the stronger player ... Ended up eliminating him too. Lol
"I work for the company. But don't let that fool you, I'm really an okay guy."