OK, we could stop SETI, but what else are we supposed to do? Should we turn out all the lights at night, so it looks like nobody is at home? While this would aid our environment, I don't think it would save us from alien invaders. Presumabley these advanced aliens have super-duper telescopes or robot scouts who search out good planets for them to conquer. If we are lucky, Jupiter and Saturn will hide us or at least divert attention by their bigness. If we are not lucky, then we will probably have to just surrender.
Where can this lead but to a new government agency devoted to anti-alien affairs: the Homeplanet dept. for Interplanetary DEfence (HIDE). They will spend jillions of dollars developing a whole-Earth cloaking device.
Klaatu and his Earthling helper
I'm hoping Hawking's comments will revive the long-defunct Extraterrestrial Anti-Defamation Organization (ETADO). I used to enjoy their website.