Don't tell them anything they shouldn't already know.
Don't show them anything they shouldn't already know.
Don't show them sources they won't/don't trust.
Show them their own literatrash and ask them to explain it.
Ask them questions.
Don't answer their questions unless it suits your purposes.
Don't accept diversions.
Give them guilt trips for any devious tactics or bullying they try on you. Double standards are fair game here.
Ask questions to highlight dishonesty and double standards, so that they are defending their actoins, and you are not you defending your statements.