If it's RP'd properly you can ram somebody. Make sure you don't PG tho, full /me's and /do's are needed. Once rammed over you must move off them quickly else it is carparking (you can't kill them or lower their health by parking on them).
Driving someone over in real life wouldn't probably kill them at an instant, they'd be laying injured under your vehicle. In SAMP though, if you're under a vehicle - your health keeps going down until you die, and that's why it's not allowed.