How to remove BOOTCAMP partition?
When you say "going to bootcamp assistant -> continue" do you mean you did something similar to the steps in this Apple support answer? Note that there are different steps depending on if you have a single or multiple internal drives.

If you are using the Disk Repair Utility, have you tried booting into the Disk Utility (hold down command+r when you boot the computer or restart)?

Especially if you have a single internal drive you may also want to create a bootable USB drive, and then boot to it to use Disk Utility to alter the boot drive partitioning (so the drive is not reading and writing to the same physical media). Holding down option when you start allows you to select the startup drive.