It is against the law. It's taking the law into your own hands. You go back to court. You can, with time, have wages garnished, or even have him jailed.

If your in California and there is a court order for visitation then you can be arrested for witholding the child. Child support and visitation are very different subjects and are treated as such by the courts.

It is against the law to violate a court order. If there is an order allowing the father visitation, unless that order is contingent upon his paying support, which would be very rare, then it is not grounds for violating the visitation order.