Did you actually ADD mobs to mongen, or are you just using the mongen that came with the server files?
The mongen included with files only has PB and some easter maps lol.
Either that or you have wrong Mob names in Tbl_mongen, wrong map numbers in Tbl_mongen, bad co-ords in Tbl_mongen, or wrong race/raceimg in Tbl_Monster.
As far as I'm aware, there is no DB setting to eliminate spawns besides having 0 in FLD_Load in Tbl_mongen