Banning Game Materials

Earlier today, I was surfing the Paizo forums and I found this thread about banning certain game materials from their home games. Reading through the thread and seeing what things people ban from their games got me thinking.

Why do we ban certain things from our games?

Generally, there seems to be two major reasons why someone bans something from their games: Mechanical Reasons and Flavor Reasons.

Sometimes, a game master will ban an options they feel is mechanically unbalanced or fear will screw up the mechanics of the game in some way. This tends to be the reason most people ban the Summoner class. Because of the class' complexity, its relatively easy to create an eidolon that is either horribly useless or broken if you don't know what you're doing or rather overpowered if you do know what you're doing.

Others ban certain materials and options due to their flavor and how it might not match the game master's current campaign setting. This seems to be the reason why most people ban the Gunslinger, the Ninja, the Samurai, and certain races. For example, some game masters ban the Gunslinger because they believe firearms have no place in fantasy worlds or blackpowder weaponry just doesn't fit the setting they are using.

Personally, I have a softer view on banning certain materials from my game. I generally allow pretty much anything that is presented in the Core Rulebook unless noted otherwise (I wouldn't allow players to play gnomes if they don't exist in the campaign world, for example). However, if they want to use something from a supplementary source (whether it be from Paizo or a 3rd party publisher), they have to present the material to me and let me look at it first. I will usually allow them to have the option, but if I believe its broken, overpowered, or clashes with the setting, I'll say no and give my reasons for rejecting it. This method has worked for my group so far.

Question Time: Do you ban certain materials from your games? If so, why? Is it due to the mechanics, or is it for flavor reasons? Leave your answers in the comments below.