You can't disable the WEP key, you just untick the WEP option.
Go back in to your router's configuration utility just as you did to set up WEP.
Go into the wireless section and disable WEP (untick).
Can't be more specific because it depends on the router make & model which you haven't disclosed.
Basically, if you've set up WEP encryption already you should know how to turn it off.
Bear in mind that WEP protected networks can be easily hacked, and with no encryption at all your network can be used by anyone who's in range.
Use WPA or WPA2 for better protection against freeloaders and hackers.