Scripting only method. You could just put something like this in your mods "OnPlayerLevelUp" event:
void main()
{
object oPC = GetPCLevellingUp();
int iXP = GetXP(oPC);
if (GetLevelByPosition(2, oPC) || GetLevelByPosition(3, oPC))
{
SendMessageToPC(oPC, "Multi-classing is not allowed. Please re-level with your original class.");
int iLevel = GetHitDice(oPC);
int iDelevelXP = (iLevel-1) * (iLevel - 2) * 500;
SetXP(oPC, iDelevelXP);
DelayCommand(1.0, SetXP(oPC, iXP));
}
}