There two places where "Increment revisions" are set. One as you mentioned is on folders. The other one is on workflows. Verify if "Increment revisions" is checked on workflow that is dealing with documents on library folder.
You can set a condition on the transition in the workflow that increments the revision to not allow it to occur if in a particular folder. You can create parallel transitions of the same name if you want the behavior to be different for groups or individuals and permit the groups to use the appropriate transition.
Thank you gentlemen for your help. In fact, Mr. Ron suggested just what I needed exactly. I added a condition to transition element, which is "File path" "Text contains" "Projects", and now in the "Next revision" window I get a warning for a library parts (which are in another folder) called "Conditions for transition were not met". This warning is not allowing me to change the revision for these library parts.
I could have added "File path" "Text does not contain" "Library folder" just to make it double sure.
Thank you for your help !