Custom columns can be deleted as long as they are not in use. Once a column is used in a media plan, deleting the column is not allowed as to avoid accidentally deleting existing data. Columns must be removed from each existing campaigns to allow deletion.
It is better to deactivate if you no longer wish to use a particular column in future campaigns. This will ensure retention of historical data.
Within the appropriate column tab (Money or Other), place checkmarks next to each column you want to delete
Press the Delete button
For Other Columns
Other columns have a 2nd option for deleting
Click on the Column Name to Edit the column
Press the Delete button at the bottom of the Edit window