Does anyone know a way to do this? I would like to metalise the unused areas and tie them to ground. I've looked through a lot of the options and none of them seemed to help!
What you are looking for is called a polygon pour. You can specify the net that you want the copper connected to, the space and shape around traces and pads connected to other nets, whether or not to remove unused copper, etc.
Look in the help files for Polygon. If you want to exclude areas from copper, just set up keepout shapes, outlines, etc - the polygon will avoid those areas.
When you choose "place polygon pour", you will see on the popup menu two choices - one is grid size, and the other is trace size. Choose a trace size that is just slightly larger than the grid size, and you won't have any gaps in the fill. The traces are centered on the grid, so if they are smaller than the grid, you have an open mesh instead of a solid fill.