COMMUTER FLT LNDG PRC INSTRUCTED TO USE RWY 21L IGNORED CTLR'S INSTRUCTIONS; OVERSHOT THE RWY 21L FINAL; AND HAD AN NMAC WITH TFC OPERATING ON RWY 21R.

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ACFT X; A COMMUTER WAS ON A VISUAL APCH. ACFT Y; A C172 WAS DOING TOUCH AND GOES RWY 21R. ACFT X ENTERED DOWNWIND RWY 21L AND WAS SEQUENCED TO FOLLOW A C172 LNDG RWY 21L. ACFT X RPTED TFC IN SIGHT AND WAS CLRED TO LAND RWY 21L. AS ACFT X TURNED BASE HE NOTIFIED THE TWR HE WAS TOO CLOSE BEHIND HIS TFC AND NEEDED A R 270 DEGS TO FOLLOW. I INFORMED ACFT X THAT 'A 270' WAS UNABLE; 'A 270' TFC WAS USING RWY 21R; CTRLINES ARE 600 FT APART. I TOLD HIM TO CONTINUE RWY 21L AND IF THERE WAS INSUFFICIENT ROOM HE COULD EXPECT A GAR. ACFT X THEN OVERFLEW FINAL RWY 21L ON WHAT APPEARED TO BE A WIDE S-TURN. REINSTRUCTED ACFT X TO NOT CONTINUE W AND TOLD HIM THERE WAS TFC. ACFT Y WAS ON R BASE FOR RWY 21R WHEN ACFT X CONTINUED ACROSS FINAL FOR RWY 21R. I ISSUED A TFC ALERT; A GAR AND A CLB TO ACFT X. ACFT Y TALKING TO ANOTHER CTLR WAS ISSUED TFC AND DID AN EVASIVE TURN TO THE R WITH A DSCNT. 1) ACFT X IS A REGULAR FLT -- IGNORING RWY 21R IS DANGEROUS. 2) PLT APPEARS TO HAVE COMMITTED HIMSELF TO THE OVERFLT OF FINALS EARLY -- BAD IDEA. 3) INSTRUCTOR PLT HAD GREAT AWARENESS: ACFT Y.