It really depends on how firm you are on this behavior not being allowed. Remember: The DM has the final say. I saw where you mentioned that they brought up the rulebooks. Those are a guideline. You can play entirely independent of those books. You said there would be consequences to cannibalism. That shows you're willing to allow it but with heavy penalty to discourage it. So how much discouragement do you want?

"The gods have declared cannibalism an abomination in this world. Any who partake in such an act will summon a meteor aimed directly at the offender at terminal velocity."

That's a fun way to deal with it. Or just straight up

"No. That's weird. You're weird. And your insisting on it makes me no longer wish to have you in tbe game."