Marina Acton -- who plunked down $17.8 million in November for Kim and Kanye's mansion -- is being accused in new legal docs of harassing Deeona Capital. Deeona says Acton has been downright vengeful, cutting off the water and power, bullying her and threatening her family.

According to the docs ... the harassing behavior's been going strong since late March, and the woman claims Acton and her cronies have also damaged her personal belongings and tampered with her mailbox and Internet connection.

Here's where it gets stranger ... sources close to Acton tell us the woman -- who filed the legal docs asking for a restraining order against Acton -- is her former assistant whom she fired and who has now turned into a squatter.

Our sources say Marina offered to rent the place out to the alleged tenant for $55k per month -- but hasn't received a penny -- so she's ordered Capital to get the hell out. She's even called the police but so far Capital, her spouse and sister won't budge.

We broke the story ... Kim and Kanye sold the mansion to Acton, a rich Ukrainian philanthropist, for a huge profit.