Mark uses the computer in the course of performing his job duties. Every so often, Mark will use his break time to play games on the computer. You've never seen him playing computer games on company time, and it doesn't seem to be affecting his work or the work of other employees. Do you tell Mark to stop?
Asking him to stop is probably the wrong approach. Work rules that get too restrictive tend to have more of a negative effect than a positive one. If Mark is doing it on his own break time and he's not bothering other employees or tying up the computer, there's probably no harm in allowing him to do it. However, you might want to make a mental note to speak to Mark about it if the game playing gets excessive.
And if you do ask him to stop, be sure that all other employees who may be playing computer games on break time are also asked to stop as well.
The better path to take is to let it slide. If Mark is a good employee who always gets his work done, so long as his break time games don't disturb other employees, tie up computer resources or get excessive, allowing him to play games on his break time shouldn't cause any problems. Be sure to cut other employees in the same situation the same slack though, in the interest of fairness and consistency.