Hi,
Yes the automatic propagation is a royal pain.
You can turn it off in your job: rght-click on the cvanvas and choose Automation Settings --> Automatically Propagate Columns. Do the same for Automatically Map Columns.
The default for any new job is set in Tools --> Options --> Job Editor tab. There are some settings grouped in a box called "Automatic Settings". There you can turn off automattic column mapping and propagation permanently.
Hope this helps,
-- Jan.