THE FLT WAS RTRNG FM A VFR GLIDER SENIC FLT. THE PLT EXTDD THE SPOILERS TO LOSE ALT WHILE ON BASE LEG FOR RNY 19. WHEN THE GLIDER REACHED THE DESIRED ALT, THE PLT ATMTD TO RETRACT THE SPOILER CTL HANDLE, BUT IT WAS JAMMED IN THE EXTDD PSN. THE PLT & PSGR MADE SVRL ATMTS TO RETRACT THE SPOILER CTL HANDLE, BUT WITHOUT SUCCESS. DRG THIS TIME, THE PLT FLEW TWD THE APCH END OF THE RWY, BUT WITH THE SPOILERS EXTDD, HE WAS UNABLE TO REACH THE RWY. THE GLIDER COLLIDED WITH A DIRT EMBANKMENT SHORT OF RWY 19. A WRECKAGE EXAM DISCLOSED THAT THE FIBERGLASS SPOILER EXTENSION BLOCK WAS EXCESSIVELY WORN & ALLOWED THE SPOILER TO EXTD BYD ITS NORMAL VALUE. THIS CONDITION ALLOWED THE SPOILER TO JAM ON THE FENCE CAP RECESS ON THE UPPER SURFACE OF THE WING. NEITHER THE FLT NOR MAINT MANUAL ADDRESSED INSPN OF THE SPOILER ATTACHMENT OR EXTENSION LIMITING BLOCK. THE GLIDER RCVD AN ANNUAL INSPN 4 DAYS BFR THE ACDNT. THE INSPN WAS PERFORMED IAW THE MANUFACTURERS DIRECTIVES.