A: "He who smelt it dealt it."
B: "FYI, The draft in the office comes from your direction."
C: "Considering your breath, I think you'd be used to the aroma."
D: "The bathroom is broken, it happened the last time you planted your fat ass on the toilet."
E: (If the person is a woman) "Honey, it's not me, try closing your legs."
F: (if the person is a male) "Honey, try actually touching the toilet paper to your ass when you wipe."
Or how about the plain and simple...
G: "Grow the fuck up!"