Avoiding noise coupling by power switching to ground plane in multilayer PCB
As suggested, you want to separate your ground plane into regions to reduce noise coupling between the different functions of the circuit, in your case: switched supply and signals. The regions should be connected with each other in a single point only (see image below).

Google Image for "pcb ground plane switch" gave the following result:



(Note that the borders between the ground plane regions apply for ALL the layers. If signal traces run over multiple ground planes you might get large current loops.)

To sum up, you should go for a combination of 1) and 3).