We got Jessica -- flanked by fellow co-star and bf Cody Nickson -- outside Clifton's Thursday night in downtown L.A. for the "Big Brother" cast party and our guy tried digging about her controversial schtick -- approaching unsuspecting houseguests from behind and surprising them with a 2-finger checkup.

Jessica's having none of it ... so fed up she stops the autograph sesh and takes it inside.

It's all fun and games ... until someone starts poking.