An OnlyFans star says she's not backing down from billboard vandals ... 'cause she plans on keeping her ad promoting her account up, even though it's been vandalized three times.

We spoke with Chloe Amour -- a porn star with a new billboard in L.A. -- and, she tells us she can't catch a break when it comes to incidents of her image being slashed.

Amour tells us Regency Outdoor -- the billboard company that keeps putting up Chloe's ad -- told her they suspect religious protesters might be behind the vandalization ... which she says means it ain't about her as much as what she does for a living.

Chloe admits she didn't research the neighborhood for specific religious orgs before she agreed to advertise there -- she just picked an area she recognized off a map.

Besides, the billboard itself is tasteful, Chloe says ... adding she'd never put up a "slutty" photo of herself in an ad like this.

Despite the protests, Chloe says she doesn't plan on backing down ... and instead, is working with Regency to make sure her billboard keeps going up.  She doesn't want to upset anyone, she says, but she's not giving up.

She's also got a pretty funny anecdote about cops trying to catch the alleged vandal the third time it happened ... take a listen for yourself -- sounds like Spider-Man ain't so friendly in this neighborhood.

We also spoke with Ronak Shah from AdTime Marketing ... and, he tells us his company regularly works with OnlyFans models to get their names all over the U.S. -- with a particular emphasis on L.A. and Las Vegas.

Right now, Ronak says there's no cap to how many times they're willing to put up the billboard ... though, eventually, he admits they'll reach a point where it doesn't make smart business sense to put it back up.

Shah adds that their billboards often get graffiti ... but not this level of apparent repeated targeting.

Worth noting ... we reached out to cops and they confirmed they're investigating -- though they wouldn't say which religious groups, if any, are suspected.