Just let people cheat in single player, it does no harm....
If its due to save / load systems, like I said simply disable the player from saving in singleplayer as then they can cheat all they want but it will not make them better in the multiplayer (how good maps like SWAT Aftermath do it).
If it is to stop people exploring your map, you are just asking for people like me to rip your map appart. I can discover all secrets in a map in under 30 minutes via reading the unprotectable trigger script and I make sure I tell everyone I meet online in the games about them. Do not take this as a threat or anything, it is to point out how feeble disabling singleplayer is on hiding secrets and in the end will ruin your map if implimented for this reason.
The easiest way is on map initialization, check if there are more than 1 human player in game, and if not then disable the save / load system.