Two flavors are in play here. Some players take a long time to go back to the chair when their turn is over. Others are disrespectful, moving or making noises while in the chair.
Opponent is slow to go back to the chair
If there's no shot clock in play, just wait until opponent sits down before approaching the table. In most cases, opponent will realize that their gamesmanship isn't paying any dividends, and they might well stop trying to shark you in this manner.
The real problem is when opponents are slow getting back to the chair when a shot clock is in use. I've seen it happen often at pro level and it is a practice employed by a couple of very elite players whom I prefer not to single out. In theory, the shot clock shouldn't start until a player approaches the table, but in practice this is not how it ever goes down and one of the players is invariably shortchanged on the shot clock.
Opponent is disrespectful while in the chair
If, through noises or movement, opponent is sharking you while in the chair, ask them as politely as possible to sit still and quiet while you shoot, and if that doesn't work, get the tournament director. In a social or gambling watch, you might be out of luck, but you can choose which players you gamble with or play socially, so steer clear of those who shark you.