What to do about slander?

[mikesm8]

My husband has been coaching for 10 years. He helped a couple coach and the relationship ended not so nicely. For the remaining of his volunteering, he continues to coach this couples children. The mother of the children constantly sits in the stands and spreads her negative opinions to the other parents. She states what horrible coach he is and how he shouldn't be coaching. This has went on for a couple of years and we have ignored it. The straw for us was last friday's game, she went to the dugout and told the manager of the team my husband was coaching the my husband can't be on the field or in the dugout because he called her daughter a "F*** B*** and she had already went to the Baseball Board and this was their ruling not hers. My husband called the President of the Board and he claims he has never spoke to anyone about this issue. This statement is completely false and we just want it to stop. The situation is escalating. What can we do to make her stop and do we need to file a police report for this last incident if we do want to file legal action?

[Mr. Knowitall]

If the baseball board has that type of power, why not explore this option of having this couple banned from attending games or practices?