QB rule question

I was just thinkin about this mostly cuz of all the ridiculous rules we have now to protect the QB... what are the rules for when players CAN hit the QB after the play?? Seeing as Brett Farve ran 20 yards or so downfield to throw a low block on Patrick Willis a few weeks ago shouldn't players be allowed to hit the QB too?

I'm not talkin about running up behind a QB and going low on him or cracking him when he is not looking, but if he is allowed to run up and throw a block on a player downfield what is to stop someone from obliterating a QB when they see him running in the open field without the ball. I think it would get flagged but it's a bit hypocritical.

Anyone know the actual rules?

If they're running around making blocks, they can get hit just like everyone else.

As a QB, you have to treat them like a delicate newborn baby.

As a RB, see above if they slide feet first. If they dive head first see below.

As a blocker, it's fair game and you can knock them out to next week!

basically, when it comes to the QB, it's

"NO TOUCHING!"

You make sure you pull the flag . Oh wait..