In theory you can send an aircraft smack into the side of a mountain just outside controlled airspace.
In practice, where I work at least, we don't do that, we usually clear aircrafts to leave CTA at a safe altitude, that will also remain safe for a while after they exited the CTA.
In the few occasions where pilots absolutely want to leave CTA at altitudes that aren't "safe" outside, we advise him of the possible bumpy ride, and make them understand it's their responsibility to not crash outside the CTA
There has been grim accidents from misunderstandings from pilots about who is responsible for terrainclearance amongst others, so here we like "nursing" a little to be on the safe side of things.
Same goes for danger and restricted areas, we don't send them directely towards them, and just "terminate radarcontrol", we'll as a minimum advice them, even vector them around if requested.