Telling ya, fill it with something flammable and them flick a match!
I use to work with someone who had this same problem one time. It was some elderly lady who use to have a pool in her back yard. One day ducks showed up and she was happy about it. She made the mistake of feeding them ONCE, and they never stopped coming back. After about a week of them crapping and fucking in her pool, she got sick of it and tried to scare them off.
At first they were afraid of her, but that only lasted for about 2 weeks. Next she started to throw stuff at them like you, but they would always come back. So one day she bought a vicious pitox that was oddly enough trained both as an attack dog AND a hunting dog. She would let the dog out whenever she saw the ducks, but he could never catch them. One day she came home from work and found her bedroom window broke and the dogs head in her bed.
She was scared shitless and pissed off at the same time. She loved that dog more then moth loves fire and centipedes, which is why she was angry. But she was scared because she lived in a bad neighborhood and had CAST IRON BARS OVER HER WINDOWS! The ducks broke through the bars and window, just to put her poor dogs head in her bed.
Well, she took that as a last straw and kind of mentally broke. She soon quit work so I only got to talk to her outside of work after this. But apparently she bought a flamethrower and tried to torch the suckers. She got the female one one day, but the male got away. Next week she vanished. No trace of her, just a bed full of duck feathers.
True story.
Burn them now, dont mess around with them. You mess with them, your as good as dead.