Open the Misc tab of GameOptions and adjust the map scroll sensitivity zone to your liking.......I like 25 but you may prefer 10 or 12 .. Zero turns map scrolling off altogether which allows you to only scroll with the keyboard.....I don't know if anyone actually goes that far though.
This is explained on page one of the game quide'
Map Scroll Sensitivity Zone- Sets how close to the edge of the map window the auto-scrolling starts to move the map. Set this to 0 if you do not like mouse map-edge scrolling. (NB - Mouse scrolling is not checked for in the edge portions of the map covered by the mini-map display).
Don