Sex offender in extended family network

Sylvia, your humble servant here has a convicted sex offender in his ex-son-in-law's family.  The brother of the ex-son-in-law molested his own children and spent time in prison for it.  His wife and kids rightly divorced him.  When we send our grand kids to visit with "poppa," the divorce decree says this offender is not to be present, ever.  He is, and if it continues, grandpa here may take legal steps to slap down this behavior good and hard.  Dr. Rux