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Open the BootCamp Assistant, select "Create or Remove a Windows Partition" click Continue and follow the wizard.

Backup any data you want first, once its gone its gone.
 
will this remove all data from the hard drive or just the data from the boot camp partition?
 
It removes the partition and then extends the Mac HFS partition to take up the "free" space. If you are looking to "destroy" the data for some reason, you can then run the Disk Utility and have it erase free space on your Mac partition with multiple passes.
 
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