It's still OK to block them, just not check them off the ball, if they run into you that's still OK, the only thing it's trying to get rid of is people pushing 2 defenders off their bikes as the 3rd guy gets a free shot on goal. I'm sure it's OK to ignore a little bit of incidental contact, all you need to do as the attacker who is screening is to get in the defender's way and be ready to receive contact if the defender tries to get past you with contact rather than speed or out braking.
It's still OK to block them, just not check them off the ball, if they run into you that's still OK, the only thing it's trying to get rid of is people pushing 2 defenders off their bikes as the 3rd guy gets a free shot on goal. I'm sure it's OK to ignore a little bit of incidental contact, all you need to do as the attacker who is screening is to get in the defender's way and be ready to receive contact if the defender tries to get past you with contact rather than speed or out braking.