In Tools > Options > Audio, there is an option to set maximum volume, and you can also enable Volume Protection which changes the volume to 20% at the start of playback, and limits the rate at which it can be increased.
I'm not sure that there's a way to never allow Windows to go over a certain level.
I don't think there is a way to hard-limit windows; if you're only using JRiver, the max volume setting works great. If you're not only using JRiver, I'd recommend a passive input attenuator.
You can easily get 10 dB in-line attenuators built into an XLR sheath for ten or twenty bucks a piece.
You'd need about 6 dB of attenuation, not ten, but depending on your set up . 10 dB should be about right, but you may want a little more attenuation as a "safety". That is to say, you may not want the full rated output of the amp, or your sound card may put out slightly more than the rated 2 volts (I had a soundcard rated to 2 volts of output that put out 2.15).
Input attenuators also usually have the beneficial side effect of lowering the effective noise floor of your sound card/DAC, so there's that too.
EDIT: Made some fixes, when I said previously that you'd need 6 dB, I was thinking of power attenuation, not voltage attenuation; you probably do want about 10 dB of input attenuation, which is convenient because there are lots of comparatively cheap in-line attenuators in the 10 dB range.