Two things. Absorb, as Menilus mentioned. That power blocks most of the other force powers.
It also depends on the amount of points that both you and the other guy allocated to push and pull. Pretty much, if your opponent has more points allocated to push and pull then you do, they'll block it. Or if you have your push and pull filled with all it's points, but he has his push and pull, he'll still block it as long as he isn't doing anything else. In that case, to pull off push and pull you gotta wait until either:
A) they're using a force power
B) they're jumping in the air
C) they're swinging their saber or firing their weapon
Those three things distract their defenses and thus are optimal times to use the push or pull powers on them. I'ts all about timing.