I am a novice Game Master directing my first game.
My players have recently discovered the mage in the group can cast "Create Memories". The spell reads:
Permits creation of new memories in the target, without necessarily erasing the pre-existing ones. Most of the time, unless very deeply-rooted memories are modified, the person affected will feel slightly confused, but will be convinced that his memories are real. The caster determines what information he is trying to introduce, regardless of its complexity or duration.
The spell allows for a save, but typical level-0 peasants need to roll 80+ on a d100 to beat the difficulty, so most of the time they will be affected. It however allows for an extra resist throw each time their fake memories are contradicted.
The problem is that they are now using it to get anything they see that they want. For example, they see a nice sword at a weaponsmith's? They change the memories of the owner so he remembers that they already paid, and just walk out with it.
How should I deal with it? Should I just let them take anything not heavily guarded like that, and have more important things guarded by more resistant NPCs? Or should I do something else about it?
Some background on magic in the setting and people's reactions to it might be helpful. Casting does require some gestures to be performed, so witnesses will notice it, especially since magic is feared in my setting. (People know what it is, but will call the inquisition as soon as someone does something resembling magic.) If it is a small group of people, they might hold them up while the mage changes all their memories. Any guards seeing it will become really aggressive if they see it.