You have multiple methods/solutions that you can use here.
1. hide your playable character & use an npc version.
2. you can enable/disable left click from setting destination with a line of Lua script.
game.LeftClickBehaviour = eMouseActionBehaviourDoNotSendCharacter
These are the available options...
'0' (eMouseActionBehaviourDoNotSendCharacter): The character will not be send to the current position.
'1' (eMouseActionBehaviourSendCharacterToCursor): Character will be send to current position.
'2' (eMouseActionBehaviourSendCharacterToObjects): Character will be send to current position if it is a scene object.
Quick note: if you use the scripting method, it will prevent left click from setting a new destination, however if you click on a scene object that contains an immediate execution type action & that contains action parts that update the destination position/object, then the character will move.
3. you could change to an outfit that has a walk speed of 0, but that method is probably more hassle than it's worth as the character will be trying to change to the walk animation & won't actually be able to move anywhere.