Is the child capable of saying more than "stop?" How well does this particular child understand conversation about social behavior?
If it is possible for the child to have a rational conversation with an adult about social behavior, then I'd recommend a private talk. In the talk I would find out everything I could about what was causing the child to shout "stop" - but it's likely a control issue at that age. It sounds like the child is the youngest in the group, so it is likely a feeling of being left out that is driving the demanding language. If at all possible the child should be led to voice the exact issue to you: "They never let me choose what we'll play" or whatever it is exactly.
Once the child can voice the problem beyond that single word, "stop," conversation can progress to appropriate social behavior including strategies for improving the situation. Go through scenarios with him, lead him in practicing what to say and how to say it to gain better success. If it is found that the child is powerless in spite of good effort, then the other children must be brought into a separate conversation about their social behavior - and it sure does sound like that will be needed.
If the child can't communicate properly, he won't be able to play well with others, and perhaps then the others ought to be chosen carefully for their willingness to accommodate this child (which would be necessary for certain forms of communication disabilities). But it doesn't sound like this is your problem.
Private talks and conversation modeling are a big help in our house