I found that I had to adjust the settings for cec for my samsung tv. Follow this blog to update your remote to include all the buttons:
http://blog.jl42.de/2013/01/changed-rem ... h-raspbmc/
After I updated my cec remote mapping, the blue button on the samsung remote was what I needed to turn subtitles off. It appears to be on by default, and there is probably an option somewhere to go turn them off by default.
This is what I have in my remote.xml and does all the functions I've needed so far:
<!-- Configuration file for driving CEClib remote -->