There are a number of cleaners specific to cleaning spark plugs; they are self contained blasters. There is also some blast media specific to cleaning plugs. A fine wire brush and some carb/brake cleaner works wonders, and a high pressure blast of such fluid followed by high pressure air blast should remove all traces of media.
If you think you cannot remove the media in these methods, then try the finest walnut shell media you can find, OR soda blast media. The latter will dissolve in water, the former will not be an issue if traces remain.