Here's the deal ... TikToker Hunter Echo recently alluded to having a sexual relationship with Millie, claiming she was in love with him and saying he thought her parents knew about the alleged relationship because they were all living under one roof.

Millie's reps tell TMZ ... "Mr. Ecimovic’s remarks on social media are not only dishonest, but also are irresponsible, offensive and hateful. Instead of engaging in a public discourse with him through the press or on social media, we are taking action to ensure that he stops this behavior once and for all."

The reps for the "Stranger Things" star would not say specifically how they feel he's being dishonest.

Hunter came under fire for his crude comments seemingly aimed at Millie -- he mentioned tons of kinky sex acts -- but he says he won't apologize for anything.

We reached out to law enforcement sources across L.A. ... none of which had any sort of investigations open concerning Echo.