How to format "free space" on a drive?

I bought a 2 TB hard drive and created a 1.5 TB partition in ExFat format. The other 500 Mb was left as "Free space". I thought that I could format that free space later, but now I don't see a way to do so. When I select that partition in Disk Utilities all the options seem to be grayed out. Is it possible to format "free space"?