Jacky -
In general I do not recommend doing this, but if the name of the variable is unique, you might be able to delete the variable from the file by closing the file, removing a line from the INI content of the file, and reloading the file. Make sure that you back up the file before hand editing the file in case it does not work. For example, remove the second line shown below to delete the MyFileGlobal variable.
[DEF, SF.FileGlobalDefaults]
MyFileGlobal = "TYPE, Error"
Once you do this, you will have to reconstruct the variable the way you want it.
Scott Richardson