Tips on dealing with MIL

Any tips on how to tell MIL not to kiss my baby without being too offensive? This MIL we had told her through SIL but she obviously disregarded what we said to her and purposely kissed my baby on his lips. I was so disgusted by her. Out of respect (even though she was disrespectful to our wishes but I don't want to stoop to her level), I want to tell her directly not to do it during our next visit but do not want to come across as rude. Appreciate any advice! Thanks!