"Real Housewives of Beverly Hills" star Dr. Paul Nassif claims his disgruntled former nose job patient tried to shake him down for money ... threatening to trash him all over town unless the doc forked over a big, fat check.
TMZ has learned ... Nassif has obtained a restraining order against Roman Melikov ... claiming the Roman threatened him and and his family because he was unhappy with a recent schnoz job.
Now, sources close to Nassif tell us ... the doc believes Melikov tried to EXTORT him too ... AFTER the two sides had already worked out a settlement regarding the procedure.
Sources close to the situation tell us ... Nassif shelled out $3k to make Roman go away back in March 2011 ... and Roman accepted the deal.
But, we're told, Roman -- a self-proclaimed portfolio manager -- then changed his tune and told Nassif he would "have no choice but to do negative reviews" unless the doc either refunded the entire $26k bill or invested $400,000 with Roman's firm.
Nassif refused ... and instead threatened to sue Melikov for defamation.
Roman fired back at Nassif in an email ... telling him a lawsuit would be pointless because he has no assets and $82k in debts.
We called Roman for comment, but never heard back.