After the fords have become a bridge? Impossible, AFAIK.

Before the start of a new campaign, you could try annd fiddle with the Map_Regions.tga. Try to give all fords to the Eleutheroi province of Terhazza. (Use the same colour as the one used to paint the Terhazza region.) Since no roads can exist in the Terhazza region, no roads can
exist across fords, hence no bridges can exist.