Locked Document Problem

Alright I've used Excell for several years, mainly for spreadsheets at work and have never had a problem with it being locked. However I've been training a new guy at work and somehow when I'm not THERE watching him... he manages to make a cell into a fraction and because the document is locked, I can't fix it properly. I've searched on google and gotten nowhere really, any help is appreciated.

I've tried deleting the cell and inputting the information and it still goes back to being a fraction, I am stumped. Is there a way to convert this back to decimal without having to unlock the document?

The formula I'm using is -1709./107 which comes out to -15.97, but Excell is putting it as "-15 69/71" which is still the correct number, but it ruins my spreadsheet entirely because other cells are linked to it. I've also tried only entering -15.97 but Excell still displays the fraction, getting rather annoying.

Not sure how when the cell is locked that this guy can get it to do this...

Have you checked the formatting on the cell?

I can't format it because it's locked, the option is not available, that would have been the easy solution yes.