I used to sync all my Iphone photos via wifi to ubuntu one.
I just wanna stop syncing photos from my iphone now and esnure that all the space that was previously used for this on ubuntu one is recuped and made available for other files
Go to U1 app on your IPhone, select Uploads tab then uncheck Auto-Upload Camera Roll. After go to Ubuntu One page and from Files tab select More on the your photo's folder and click Stop sync. Also read FAQ its kinda helpfull.