JNU CTLR DESCRIBED CONFLICT WHEN GAR; LNDG CLRNCS WERE UNCLR AND ACFT MADE UNEXPECTED MANEUVER TO LAND ON RWY 8 BEHIND DEP TFC.

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I WAS WORKING LCL CTL AT JNU ATCT. I PUT ACFT X (A C150C) INTO POS AND HOLD RWY 8; WHEN ACFT Y (A PA32-300) WAS 2.5 MI FINAL. TFC AHEAD OF ACFT X EXITED THE RWY; AND I CLRED ACFT X FOR TKOF. ACFT X WAS SLOW TO ENTER THE RWY AND DEPART; AND I INSTRUCTED ACFT Y TO GO AROUND AND OVERFLY THE PARALLEL WATER LANE APPROX 500 FT S OF THE RWY. ACFT Y ACKNOWLEDGED THE GAR; AND I TOLD HIM; 'XWIND TURN AT YOUR DISCRETION; RWY 8 CLRED TO LAND;' INTENDING FOR HIM TO ENTER R TFC FOR RWY 8 AND LAND AGAIN. I INSTRUCTED ACFT X 'FLY STRAIGHT AHEAD' SO HE WOULDN'T TURN INTO THE XWIND AND CONFLICT WITH ACFT Y. AS I UNKEYED THE MIKE FROM MY XMISSION TO ACFT X; ACFT Y; NOW ABOUT 50 FT AGL; MADE A RADICAL TURN TO THE N -- CUTTING LESS THAN 200 FT BEHIND ACFT X -- AND DOVE FOR THE RWY. I INSTINCTIVELY XMITTED 'ACFT Y;' THEN CUT OFF AS I REALIZED HE DID NOT APPEAR IN CTL OF THE AIRPLANE. HE MANEUVERED FOR ABOUT 10 SECONDS BEFORE STABILIZING THE ACFT MERE FT OVER THE RWY; THEN RESPONDED 'I WAS CLRED TO LAND; RIGHT?' I REPLIED; 'THAT WAS A XWIND TURN TO ENTER THE DOWNWIND; BUT YOU'RE OK NOW.' ACFT X WAS ON THE UPWIND CLBING OUT AT THIS POINT; AND IT WAS SAFER TO LET ACFT Y LAND THAN SEND HIM AROUND AGAIN. AT JNU ATCT; WE SEND ACFT AROUND WHEN A LOSS OF SAME RWY SEPARATION IS IMMINENT. HOWEVER; WHEN DEALING WITH AIR TAXI AND OTHER FAMILIAR PLTS; WE USUALLY ISSUE THE GAR; WAIT UNTIL THE RWY IS CLR; THEN ISSUE A LNDG CLRNC WITH A 'LONG LNDG.' THE PLT USUALLY DOES NOT CLB ON THE GAR WHEN HE OR SHE HEARS 'CLRED TO LAND; LONG LNDG APPROVED;' HE OR SHE THEN CHOPS THE PWR AND DIVES FOR THE RWY. ABOUT 99% OF THE TIME; THIS PROC WORKS; BUT IT PROMOTES THE PLT MIND SET THAT A GAR IS MERELY A DELAYED LNDG CLRNC. WHEN WE ACTUALLY NEED THE ACFT TO GO AROUND -- APPLY PWR AND CLB BACK INTO THE PATTERN -- THE PLT EXPECTS AN IMMINENT LNDG CLRNC AND FLIES ACCORDINGLY. IN THIS CASE; THE FOLLOWING PLT WAS SO FIXATED ON LNDG THAT HE JEOPARDIZED HIS ACFT BY PERFORMING A RADICAL LOW-ALT SIDESTEP MANEUVER IN BEHIND THE DEPARTING CESSNA; RATHER THAN CLBING; RE-ENTERING THE PATTERN; AND LNDG SAFELY. I BELIEVE I COULD HAVE BEEN MORE CLR WITH MY INSTRUCTIONS TO ACFT Y PERHAPS BREAKING THE PHRASES 'XWIND TURN' AND 'CLRED TO LAND' INTO SEPARATE; SEQUENTIAL XMISSIONS ONCE I OBSERVED THE PLT BEGINNING THE GAR. NEVERTHELESS; HAD THE PLT RESPONDED TO 'GO AROUND' IMMEDIATELY WITH INCREASED PWR; A SIDESTEP TO THE S AND A CLBOUT TO PATTERN ALT; I BELIEVE THIS ENTIRE SCENARIO WOULD HAVE BEEN AVOIDED. PLTS NEED TO REALIZE THAT WE DON'T USE 'GAR' JUST TO COVER OURSELVES ON THE TAPES -- IT HAS THE REAL PURPOSE OF KEEPING ACFT SEPARATED. LIKE WISE; CTLRS NEED TO REALIZE THAT USING 'GAR; EXPECT LONG LNDG' OR SIMILAR TECHNIQUES WILL SOMEDAY RESULT IN A PLT NOT GIVING YOU THE GAR; THE MOMENT YOU NEED IT MOST.