The scenario: Paul likes to play with Sherman’s favorite red fire truck. Sherman snatches the car out of his hands. At the same time he grabs with his other hand everything else around him. Paul has no right to touch any of his toys. Mom shouts: You must give him something to play with or otherwise he will not visit you again.
(I have to admit, I have done the exact same thing.)
First problem is you are blackmailing you child, second your child is not yet old enough to understand the consequences. Never the less it is correct to intervene. As verbal communications at this age are still limited, as a parent you are committed to help out.
Example: Paul, why don’t you ask Sherman what toy you may have?
The possibility that Sherman lets Paul have one toy is relatively high even if it is just the most uninteresting one of all. If he accomplished that first step it will make it easier for him to give up on the red fire truck 10 minutes later.
If Sherman rejects to give up even one single toy you have the chance to say: Sherman you don’t want to share a toy? – Mmh, this is silly, sorry Paul but all the toys belong to Sherman. I would like to give you some but those are not mine.
It is a way for you slow down the conflict and Sherman learns to understand that he is accepted and he has control over (the situation) his toys. Than you can try again: “Look Sherman, maybe Paul can play with the police car”?
If that does not help take Paul aside and let him play with things they don’t belong to Sherman (keys, pencil and paper etc. I won’t take long and Sherman is ready to give up on his rebellious stance because the new game is much too interesting to not join in.