Cannot delete windows partition from mac
First of all, try both repair disk (if grayed, then verify), and repair disk permissions. If you are still unable, boot to single-user mode (Command +S  on boot) and type in the following.

/sbin/fsck -fy


Press enter, then type reboot.

If it still does not work, then do the following, but be sure to backup:

Hold Command +R  on boot to start Recovery mode, select disk utility and do it from there. If it is grayed out, select the partition scheme to 1 (or 2 if you need Untitled)



Warning: There have been reports that setting the partition scheme to 1 or 2 can erase all your data. Be sure to backup before attempting to do this.