Don't "Just" Say No! Mean It.
A: What do you mean "you just can't say no"? If your mother walked into the room just before you acted out, or if someone put a gun to your head and threatened to kill you if you acted out, would you still not be able to say no?
This proves that you can say no, you just don't have enough will-power or enough of a deterrent. That is where the TaPHSiC method comes in. If you make a shvuah for a month that every time you feel you "have to" masturbate you will have to first exercise 20 minutes - and if you don't, it will cost you a painful and big knas, you will see how much less often you "can't say no".
This will help you drastically cut down. And the less we feed it, the less we need it. Over time, you will see it will get much easier to say no.