Method 1 (using Menu):

1. Navigate to menu in Configure/Controls/Assignments.
2. Remove "Prim. Mouse Button" assignment for "Fire" action and remove "Sec. Mouse Button" assignment for "Optics" action.
3. Assign whatever new action you wish on mouse buttons.
4. Join a server and try the mouse buttons. Go in Attack mode to properly test if Fire action is happening when pressing Left mouse button.

Result: Left mouse button will do BOTH Fire and the new assigned action at the same time. Right mouse button will do BOTH Zoom and the new assigned action at the same.

Expected: LMB and RMB to do the actions they were assigned to.

Method 2 (editing config file):

1. Open the .DayZProfile file in a text editor.
2. Search and remove all traces of Left mouse button (KeyCode {65665}) and Right mouse button (KeyCode {65536}).
3. Join a server and try the mouse buttons. Go in Attack mode to properly test if Fire action is happening when pressing Left mouse button.

Result: Mouse buttons will still do Fire and Zoom actions.

Expected: Mouse buttons should do nothing since they are unassigned.

There are many bug reports since December that are poorly explained related to this and none of them have been acknowledged so I tried to do my best to make a proper Report that explains clearly the situation and how to reproduce.