Is there a way to blow away the old match library and start anew?
Yes there is. Someone may come along with a simpler way to do it, and given you've made major changes in your library, First, create a new, blank library. Then turn on iTunes match (by default it is off when you switch libraries). The library should populate with grayed out tracks (everything in the cloud. Select them all and hit delete. It should pop a warning asking you to confirm you want to delete them from the cloud.
Once you have deleted it all, go to the store tab and select "update iTunes match." This will start a scan of your empty library, and send Apple an updated library database, hopefully an empty one.
While I generally give Apple's servers some time to chew and get my new "empty" database active, once I've had coffee, I re-load the original library, turn on iTunes match and let it do its thing.
I should add that it seems like iTunes Match will throw up "add this computer" as well as sometimes going all the way back to "buy iTunes Match." It also periodically asks for your Apple ID again. I just click through, add the ID when prompted, and everything seems to be fine.