You can probably do that by right-clicking on an empty part of your screen. Select "properties", select "settings" then "advanced" and then there will probably be a tab "performance" or something like that, where you can change setting of hardware acceleration.
Another option could perhaps be checking of (=selecting) "Force RGB mode" on "Video"-tab of "Options"/"Preferences".
However, that being said: maybe it is not at all necessary to turn off hardware acceleration!
I have not done so and I've got no problems whatsoever with displaying both video-files and subtitle-files both on PC-monitor and (in clone mode) on TV. Have a look
here