Use you whistle to stop the restart (rather than just sneaking a word in when you get a chance), call the player over to meet you and essentially, make sure the other 21 players on the pitch are watching you.I agree with all of the above. I am still figuring about how best to progress from a quiet warning to a public one. Taking longer over the conversation, calling captains in etc. Is there anything else that shows authority in this regard?
A public word like this is all about setting them up for a card if they slip up again - if 21 people have seen you tell a player not to do something and he then does it, the caution that follows can become a very easy sell.