Spitting!

B

My son has started taking sips of water/milk and when he's had enough he'll keep it in his mouth for a few seconds and then spit it out (more like letting it dribble out).

The first time he did it, I told him we don't do that and cleaned him up. I feel like that only made things worse; because I reacted he now thinks it's funny and does it all the time! Urgh! Any tips?