Clearing out a Cisco switch configuration is always a pain because VLANs are kept in a seperate file from the startup-config (NVRAM). There's two ways to clear a switch back to the factory defaults – the easy way and the REALLY easy way:
The easy way –
Switch# write erase
Switch# delete flash:vlan.dat
The REALLY easy way –
Hold the "mode" button on the front of the switch for 10 seconds. The lights will blink then go solid – the switch completely wipes all configuration and then reboots. Obviously, this method only works on stackable switches as the chassis based switches do not have mode buttons.