Is it this prohibitory sign?
Circle with Slash - could not load boot.efi, or some other issue
Does the Mac shut down after it shows the sign? http://support.apple.com/kb/TS3742:
If there are five new kernel panics within three minutes, the Mac will display a prohibitory sign for 30 seconds, and then shut down.
You could try repairing the disk or the OS X partition:
- Hold command-R on startup
- Open Disk Utility
- Select the disk (like APPLE HDD HTS541010A9E662 Media) and press the Repair Disk button
- Select the OS X partition (like Macintosh HD) and press the Repair Disk button
You could also try erasing and reinstalling again, but choose Mac OS Extended (Journaled) instead of Mac OS Extended (Journaled, Encrypted). You can enable FileVault later from System Preferences.
If the drive is grayed out in Disk Utility or not shown when you try to reinstall OS X, you might need to replace it with another drive.