Restore disk in single user mode

You should be able to use dd to destroy the partition's header. You can then format (erase) it and run the install.

From the installer run Terminal and type the following:

dd if=/dev/zero of=/dev/disk0s2 bs=1024 count=1024

Do be warned, this will irrecoverably destroy all data on that partition. But it looks like that's what you're expecting. Make sure you don't accidentally target the wrong thing.

Also note that I haven't done this, exactly. I've used this technique to destroy the entire partition table, not just a single partition. But I think it should work.

When you're done quit Terminal and run Disk Utility. You won't get any complaints about Core Storage.

In theory, exit code 0 means "No errors found, or successfully repaired" - though I don't know why it has then failed to complete & light the Done button.

Exit code 8 was "A corrupt filesystem was found during a check, or repairs did not succeed." which would not really be surprising if you were trying to work on the partition you were booted from.

Disk Utility is actually running a process called fsck_hfs which has published exit codes on the OS X developer pages

EXIT VALUES
     fsck_hfs indicates some status by exit value.  The current list of exit status results is:
           0       No errors found, or successfully repaired.
           3       A quick-check (the -n option) found a dirty filesystem; no repairs were made.
           4       During boot, the root filesystem was found to be dirty; repairs were made, and the
                   filesystem was remounted.  The system should be rebooted.
           8       A corrupt filesystem was found during a check, or repairs did not succeed.
           47      A major error was found with -E.

If you cannot normally Quit Disk Utility, force Quit it with Cmd ⌘ Opt ⌥ Esc , or persuade the Mac to restart or shut down normally, then you may have no option but to hold the power key until it shuts down.