You can set GPS updating to off, which means you're just flying old school off of INUs, VOR/DME updating (which you can also turn off, though those haven't really been spoofed lately). Which is going to restrict you from certain airspaces, approaches, etc. Current procedure seems to be turn off GPS updating when you notice spoofing, but while there are cues you're being spoofed, it isn't as clear as a Master Caution saying GPS SPOOF. So you fly through it for an hour and get infected INUs. You can't remove the bias from the INUs because they need a solid reference point to start from - an airport, gate, lat long, etc. I don't know of any FMS out there that lets you reenter that reference point in flight - not really sure how you'd do that given the speeds we move at.