From what little I know of the Nature2 system you should be able to remove the cartridge completely, close it up, and run the SWCG anyway. Unless, of course, they built in something that shuts the system down if you don't have a cartridge in it (to FORCE you to continue using the system) Then, of course, you can simply let the exhausted cartridge remain in the system, no longer dropping copper and silver into your water.

I would also consider that pool installer persona non grata.