Tony's suggestions re Reaper are spot on. Kenny also does a GREAT video about creating a de-esser in Reaper that can easily be tweaked to removed string squeaks etc. It's a matter, as in all De-esers and dynamic EQs, of finding the offending frequency and then allowing the compressor of EQ to suppress/reduce the problem.
I'd be inclined, personally, to use the Spectral image in Reaper and manually reduce the problem with automation of the Pre FX Volume automation line. It will take a little time but the results will be better.
Sibilant sounds and strings squeaks drive me to distraction so I deal with these things as a matter of course and self preservation.


Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe