A hit and runner is a player looking to get up money and quit playing. They are easily identifiable in heads-up games where they'll often play a few hands, sometimes as few as one, and leave up money, assuming they get up money. But they also play ring games and leave after winning a pot or a predetermined amount of money.
I welcome H&R's at my tables and would go out of my way to play them. They are generally poor players, often very poor players. They may even realize they're losing players and mistakenly believe that hit and running affords them a better chance of winning. Many hit and runners lose their entire bankroll trying to get up money. There's no guarantee they'll ever get ahead and often when they do start losing, they tilt and lose the rest of their money quickly.
Imagine if a casino were to institute a rule that you weren't allowed to play one hand of blackjack, one spin of roulette, or one throw of the dice at craps. Also imagine there were hundreds of other casinos next door that would welcome the action. The disallowing casino would lose a fortune in revenue. The reason casinos allow it is because they understand that hit and runners are at a disadvantage whether they make one bet or one hundred bets. And one bet from a hundred different H&R's is identical in terms of expected profit to a hundred bets from a single person. They also understand the H&R isn't going to walk out of the casino and never make another bet. They'll be back to try it again.
Similarly in poker, the H&R's who win almost always come back, sometimes even to play the same player. But whether they come back to play me is irrelevant, there are many other H&R's that will come to play me. And what's the difference between one or another?
Think of how many would be H&R's have sat in your games, never gotten up money, hence never given the opportunity to run, and ended up busting. You probably can't think of any because you never had the chance to identify them as a H&R. Does that mean they don't exist? Of course not, they're playing in your games and busting every day.
So embrace H&R's. They have contributed a significant amount to my poker winnings over the years and can do the same for you.