So you can't try to threaten/persuade someone into a spies-only war without turning off xp?
This wouldn't be a pre-arranged thing. I'm talking declaring war and sending a message saying spy attacks will occur and if ground attacks/cms/non-spy attacks are launched by the defender, they will face nuclear consequences; They still have the option to attack and may full well do so, but the option to retaliate with spy-only attacks may sound like the better option.
Spy attacks can still be damaging, attacking money, weapons, etc.