Use a text editor to edit the "SWATGAME.ini" file in the "content\system\" folder in the game folder. Locate the following lines:

SPDamageModifierEasy=0.5;
SPDamageModifierNormal=1.0;
SPDamageModifierHard=1.5;
SPDamageModifierElite=1.5;

Change the value for the desired difficulty setting to "0.0".

Note: Make a backup copy of the ini file before you decide to change it.