I have a few songs which I have to turn off when I hear them coming on:
'I Wanna Have Your Babies' - Natasha Bedingfield
'Manics 2000' - (dunno who sings it but I hate the bloody song)
'Hey Baby' Scooter (actually out any Scooter song here)
'My Heart Will Go On' Celine Dion (again any of her songs but this is the worst)
'The Ketchup Song'
'You Raise Me Up' - (any version, it sucks)
Anything by that guy Pitbull (what utter tosh)
Anything by Savage Garden - the lead singers voice just grated on me.
I'm sure there are tons more but these stand out for me