Remove ALL configuration on a Cisco Catalyst

Switch#write erase

Erasing the nvram filesystem will remove all files! Continue? [confirm]

[OK]

Erase of nvram: complete

Switch#delete vlan.dat

Delete filename [vlan.dat]?

Delete flash:vlan.dat? [confirm]

Switch#reload

If you can't log in and don't have a password:

Connect serial console cable to switch, press "Mode" button on the front, insert the power cord and keep button pressed until the first interface LED is lit. Release the button. Enter the following commands:
# flash_init
# load_helper
# del flash:config.text
# del flash:vlan.dat
# boot