False allegation in child custody. Can anyone shed some light on this?

Ok, to summarize: My girlfriend is in the process of a divorce for about a year now and child custody of her 5 year old daughter. We have been informed by the daughter that her dad is still getting into the shower with her naked (both naked). Her mother and I feel this is very wrong, so we voiced our concerns with the court appointed guardian. Now, the father is turning around and making false allegations against me in that I allow his daughter to see me naked! This is the furthest thing from the truth! I am really bothered by the false allegations of this man. He has no job, sits at his parents home all day while her mother and I (both health professionals) completely support the daughter. I know he is using this as a diversion tactic to get the heat off of himself with his showering incident(s), but is there anything legally I can do against him for making these allegations about me? I care about the daughter as if she were my own and would NEVER do such a thing. The idea of it sickens me and that's why this allegation is really bothering me, as a Christian man, a health professional, and as her future step-father. Is there anything I can do legally to shut this guy up from making these damaging false allegations?