Depends on other parameters.
For example if you are using Custom Movement you could do:
Spaceship - Custom movement set speed = Spaceship.CustomMovement.Speed - recoil
For bullet behavior:
Spaceship - Bullet set speed = Spaceship.Bullet.Speed - recoil
problem with bullet could be, if you have "set angle" to yes, that is you start going backwards spaceship will rotate by 180. It could be solved by additional bullet behavior.
You could also do it manually, with variables, but it would be a long story with more variables like speedX, speedY and cosinus/sinus calculation, but it could be done.
But it would be pointless if you have construct behaviors to do that instead.