is it permissable to use bbq grills that non halal meat has been cooked on?Is there a particular way to purify it, like leaving it in the sun for a few hours, or is the only way to purify it to wash and scrub all the stuck and charred material away?

You have to purify it. Wash it with water. You don’t need to remove all of the old charcoal. As long as you spray it with water. If you have reason to believe that the charcoal is najis you should remove it.