Dad sees me start to cook, comes in to the small kitchen, notices a task he can do, and gets in my way. My deterrents don't work. Ideas? He has mild cognitive impairment. He's wonderful, helpful, bored, and in my way. I need him out of my way as I have a bad knee and it already hurts just to be standing and walking. To work around him is too hard on my knee. The kitchen is very small, but there's a chair by the window and it overlooks the ocean. That goes ok sometimes, but I need new ideas please.
That can be his special responsibility.
He could also take inventory, using a form you've drafted, marking off cans as they're used so you know what to buy.
If he does that before dinner, you could tell him that he's done well and he can take a rest in the living while you're prepping and making dinner.
I think that paper shredder idea is great.
If he continues to do it guide him gently out with your hand. Eventually he'll get the hint.