if (one) can help it
If one is able to prevent or avoid something. Usually used after negative verb constructions. We shouldn't jump to any conclusions, if we can help it. This part of the country gets so hot in the summertime that no one goes outside during the day if they can help it.
not if one can help it
Only without one's agreement, only if one cannot prevent it. For example, Is he taking a second job?-Not if his wife can help it, or He's not riding on the back of that motorcycle, not if I can help it. This idiom uses help in the sense of "prevent" or "cause to be otherwise." [Mid-1800s]