I had a similar problem with word. I tried the ctrl+spacebar and ctrl+q commands. Both failed.
I then realised that 'notepad' usually removes all formatting. I simply selected all the text in microsoft word (ctrl+a), cut (ctrl+x), opened notepad, and pasted (ctrl+v). I reselected the text in notepad (cntrl+a), copied (cntrl+c) and pasted back into word
document (ctrl+v). This removed the highlighting.
Note that Ctrl+SpaceBar and Ctrl+Q clears font formatting and paragraph formatting not part of the current paragraph style. Any formatting present in the style will be left intact.
Also, highlight color is not affected by these shortcuts at all. You will have to clear the formatting by selecting the text, clicking the arrow on the highlighting button, and choosing No Color.
Pasting text into NotePad clears all formatting, which may be a rather drastic step.