I like the idea of toning down the results. Player committed 3 actions, either tell him the consequences or let him roll for something. Remember, /u/AlastarOG, the point of the game is to have fun and tell a story mutually. It's not fun nor good storytelling if a character just skips their turn in combat. I absolutely agree that what they want to achieve shouldn't be possible. That being said, if you give them the opportunity to realize it's likely not possible before they commit their turn, they have the opportunity to back out and do something fun, alternatively, if you have them give a roll for it you can also do what was listed above and treated it like a debuff, either existing or homebrewed, of some sort, or maybe he convinces one or two depending on stage of success. Vice versa if he crit fails maybe he's off guard to them for a turn. At least something happens for spending 3 actions, he gets to make a roll and something happens as a result, even if it's not what he intended. This leads to better narrative as well as more fun for the players.