There is an infinite spawning minecart that was created after using a structure block. It is lagging the map making it unplayable.

Infinite spawning minecart. Crashing map and lagging. Cannot use /kill @e to get rid of them. Pouring lava on the cart destroys duplicates but not the infinite cart. The structure block that spawned the cart is already destroyed and all other blocks brought with the structure block are already gone.