Marilyn Manson -- Sorry Lady, I Would NEVER Marry You!

Marilyn Manson has a rule -- if you lie to the media and say you're engaged to him ... he'll sue your ass.

TMZ has learned ... Manson has sicced his lawyers on self-proclaimed "rock vixen" Seraphim Ward -- claiming she leaked FALSE stories to various media outlets saying Manson asked for her hand in marriage last year.

Even though Manson's team NEVER confirmed the story, a bunch of major outlets ran with it anyway, since Ward used to date one of Manson's bandmates and she seemed to be credible.

But Marilyn wasn't amused because it was totally untrue ... and now, the rocker's lawyer has fired off a letter to Ward, demanding an apology for all of the lies she's spread about him -- including public statements she made accusing Manson of being a racist, Nazi sympathizer.

In his letter, obtained by TMZ, Manson makes it clear ... he has had "no romantic or other association" with Ward ... EVER. He believes she spread the lies in a cheap attempt at publicity.

Manson's lawyers warn if Ward continues her campaign of lies, they'll sue her ass 'til it hurts.

We reached out to Ward for comment -- so far, no word back.