Q: MAvericks disk utility won't allow me to erase

Just to confirm you are doing everything right...

OS X: How to erase and install - Apple Support

Please see the discussions in these threads:

Re: Dangerous Mission --- complete erase & OS re-install?

Re: Cannot erase my disk with USB installer

Quoting from one of those...   Have a backup of any data you care about here, as mistakes in any of the following sequences or commands can delete your data, and try the following options:

• Install OS X on an external device and try the erase from there,
• Create a recovery partition on an external device and try the erase from there,
• use a second Mac and target disk mode and try...

Key among all of these is booting from a disk other than the one that you're targeting for erasure.