There may be ways to crack the password protection of areas in MC15, but as far as I know there is no "legal" way to tamper (which includes deletion) with a locked area other than unlocking it first.
A MC15 *.xmcd is just a plain text XML file. So you could open it in a text editor, look for a region with the property is_protected="true" and delete the whole region, starting from <region .... up to the corresponding </region>
Do it on your own risk and remember to keep a backup copy of the original file!
