ORIGINAL: Dave King
Why not just delete the section you do not want after you have bounced to track(s)?
Yeah, that's how it's usually done. The reason the OP is having trouble doing what he's trying to do inside his SONAR project, is that it was never supposed to be done that way. You export the final mix as a wave file, at same bit depth and sample rate as project, then open either another instance of SONAR or a wave editor like Audacity, etc. as has been suggested, and re-import the exported stereo mix file. Then you trim the beginning (tops) and end (tails) as desired, add any fades, add any "mastering" processes in the fx bin, etc., export as a wave file at final 16/44.1 format and bingo.
I'm at a loss to understand why anybody would want to try and remove space in the project file, at the beginning or end. I mean, why?