In many pool halls, the tables are spaced away from the wall so there is just enough distance for a person to shoot normal shots. But often times, chairs and stools are aligned along the wall for railbirds and players to sit in. When a shooter plays from that side of the table, courtesy usually requires the seated person to move without being asked.
This situation offers an easy shark opportunity by the seated person. If he is a team member or a supporter, he can intentionally affect your concentration for the upcoming shot. He simply needs to stay in his chair when you come around table and get into position for the shot. As soon as you recognize that he is blocking or restricting your shot, you are forced to stop the setup, turn around, and say something. This alone is usually sufficient to distract.
Sometimes, he gets the jump on you just as you turn. With an innocent tone of voice, he asks, "Am I in your way?" When you agree, he slowly removes himself from the area. Or, just as you turn, without the conversation, he makes a big production out of moving himself aside.
Either way, he has forced your attention off the table, destroyed any tactical plans you were considering, and modified your game rhythm. With the shooting limitation seemingly resolved, you now have re-gather your focus and attention and get back into the game.
As you approach the place where you need to set up, and notice that someone is not being courteous, you can get good results by first stepping on a foot. Apologize profusely. Then, to add insult to injury, get down on the shot (before he can move) and jam your butt right into his face. If he doesn't desperately scramble away to escape confronting your backside, he certainly does so in the future if it even looks like you might consider shooting from that side of the table. The one example should be enough of a lesson to any others who might consider this trick.