Floodplain zoning - Laws can be passed that restrict construction and habitation of floodplains. Instead floodplains can be zoned for agricultural use, recreation, or other uses wherein lives and property are not endangered when (note that I did not use the word if) flood waters re-occupy the floodplain.
Floodplain building codes - Structures that are allowed within the floodplain could be restricted those that can withstand the high velocity of flood waters and are high enough off the ground to reduce risk of contact with water.
Floodplain buyout programs - In areas that have been recently flooded, it may be more cost effective for the government, which usually pays for flood damage either through subsidized flood insurance or direct disaster relief, to buy the rights to the land rather than pay the cost of reconstruction and then have to pay again the next time the river floods.
Mortgage limitations - Lending institutions could refuse to give loans to buy or construct dwellings or businesses in flood prone areas.