Playing in ranks lower than the one you belong in. Smurf players will generally be at least a few ranks higher than your own in skill, but where it gets tricky is sometimes they are intentionally losing so that they don't rank up, winning so they don't lose rank, or they are just trying to record replays to post online.

Differentiating them from normal players can take extensive experience, but the key to making them quit is dependent on their goal.

Intentionally losing smurfs: lose any way possible, including forfeits. Score into your own goal or forfeit as soon as you can.

Winning smurfs: there isn't much you can do besides focusing on saves rather than goals. They want easy wins. They will forfeit a match to get an easy win with someone else.

Replay smurfs: they don't care if they win or lose. All they care about is making it look like they scored a difficult goal. Quickly move to the goal they are moving towards and do donuts. This shows they are not actually competing against anyone and they won't be able to post their replays. If you're fast enough they will give up quickly.

If you're on a win streak of matches and suddenly it seems like you can't touch the ball, can't clear the ball, and can't save the ball, you're probably playing against a Smurf.

I only play casually so I often bait smurfs into giving themselves away. After a while I could tell within the first minute of gameplay. Most of the time, scoring into your own goal will piss them off since "winning smurfs" and "replay smurfs" are often the same people.

It really is a fun game though, and smurfs don't really become annoying until you've had a decent amount of time in the game.