Unfortunately I don't think there is a way to clear it once it is in there. Because highlighted text isn't an actual style like bold or italic for example, clear formatting or applying a different style won't work.

We had the same problem with our company templates, people wanted to use highlighted text but couldn't so were pasting in highlighted text from unlocked documents!

All we could do was unlock the document when people had finished putting their content in, remove the highlight and lock it again. Very annoying!