The suspect, James Rainford, was popped in front of Hilton's pad in the Hollywood Hills back in October after sneaking on to her property and then getting in to an altercation with Paris' private security.
Rainford pled no contest to civil assault -- and was sentenced to 3 years probation.
Today, Rainford allegedly struck again -- sources on the scene tell us he smacked Waits in the back of the head as Cy and Paris were walking into a hearing where Paris was set to testify against a different man who allegedly sneaked on to her property and pulled out a knife last year.
We're told Cy's bodyguard immediately apprehended Rainford and turned him over to LAPD.
During the incident, Rainford was telling people he KNOWS Paris ... and insisted Hilton's father, Rick, had given him permission to marry her.