[Emperor's Child]

I've yet to get to a point in a game where turtlers are a problem, but if this is the case, why not counter with another facility that can pierce the grav shield. In order to be fair to the Turtler, they should get some warning that their grav shield is being assaulted. Maybe something like it takes several turns (years?)to burn through during which the turtle-ee is given warnings.

Maybe another tactic would be to give an intel project the ability to target Grav Shield Facilities.

Again, I've never experienced a "turtler", but a on/off switch is not mutch different from building a separate grav shield facility and then halting the construction one turn from completion. This is pretty much the same effect as an on/off switch.