Can a meat grill be used to cook dairy, like pizza? If so, what must one do to prepare it for the dairy?
The STAR-K writes
One may not heat any fleishig product in a dairy toaster or toaster oven (unless it is double wrapped in foil). One may not grill dairy (or fish) on a fleishig grill unless the food is double wrapped in foil.
Regarding the second part of your question, the OU explains how to kasher from a non-kosher grill
Since food is roasted directly on the grill, the grate must be heated until it glows (libun gomur) to be properly kashered. This can be done either with a blowtorch (which should only be used by qualified and experienced individuals) or by sandwiching the grates between charcoal briquettes and setting them on fire. In addition, if the grill has a hood, the empty gas grill cavity must be kashered by cleaning, closing the hood and setting it to the highest setting for one hour (libun kal). Alternatively, one may replace the grates and kasher only the grill cavity as explained above.
Nonetheless, kashering between meat and dairy isn't simple. See here for a more detailed discussion.