How do I remove this guy from my co-op?

[YourC0mradeDodo]

So i asked one of my friends today if he could help me expand my pumpkin farm, he said yes so i added him to my co-op.
I went to the end to get some combat xp and when i came back he stole all my minions, my young dragon armor, my coins (2.5 mil) and all my enchanted items(enchanted baked potatoes etc).
After that he left and as long as he is still in my co-op I can't make any progress, as he will just steal it.
How do i remove him from my co-op? I can't kick him, and can't salvage the co-op.

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