How do I get rid of them so the AI doesn't use them?
You have to locate them manually in the folder where the game saves its designs...
Documents/My Games/Fallen Enchantress/Units
From in there you can delete the items.
You can also edit them in notepad and change specific items (if you have the patience to figure out what is what), and you can compile all your units in 1 file (copy paste) and share them with you're buddies or just share between the machines.
Hint: The AI can really use some more unit designs, so deleting them all will cripple it slightly.