One time when I had this, the solution that worked was to use a couple sims to identify more or less where the disgusted moodlet was coming from - I put them in the general vicinity, had them take one step at a time towards the suspect area, and when they got the moodlet I marked the spot where they were standing with a zero-cost floor tile. Eventually I marked out the "smelly area" pretty well. I moved everything I wanted to keep out of the way, and in buy mode, I dragged the sledgehammer tool over the area twice - once to remove the floor tiles, and once over the apparently empty ground that was left. There wasn't anything visible, but I was awarded some cash, and the moodlet stopped appearing.
This worked nicely outdoors. I'm not sure if it would be as useful indoors - you might have to do some rebuilding.
