My ex husband keeps lying to try and discredit me, any ideas how to get him to stop being such a cretin?

My ex and I split over two years ago and he continually tries to undermine my confidence by lying about what other people think of me. He previously tried to tell me that I should stop sending his best friend's children presents because they were scared of me - despite the youngest still avidly waving and saying hello to me, unprovoked. His most recent attack of slander has been to state that his girlfriends eldest daughter (11) is afraid of me, because I say hello and wave enthusiastically when he comes to pick the children up. Allegedly she has asked me to stop call at their door unannounced. I have only ever stopped at their house twice to post a letter, which I just walked up the path and popped it through the letter box, and once to drop the children off - at this time i remained by the car and said goodbye to my children there. His complete lies to try and discredit my character really piss me off, any ideas what I can do to protect from any comeback. Thanks!!!