Can I restore one partition of OS X El Capitan?
Did you try removing the disk partition with Disk Utility?  Bootcamp can be difficult with these things once you've had a first attempt.  

Did you know you don't need Bootcamp necessarily to install Windows?  You can create the partitions yourself and activate the install by booting from your USB stick image.  Just need to be sure that you have got the drivers for Windows 10 to hand from Bootcamp once Windows is installed.

All the best!