A WebIRC client is an IRC client that runs in your web browser. It enables you to connect to IRC straight from your web browser, without installing anything on your host. It can be used, for instance, to try out IRC, or to access IRC on a computer (or phone!) you don’t own.
Use one of these two, and provide your nickname and the channels you wish to join.
WebIRC users have a specific connect class assigned to them, called “webirc”. To ban WebIRC users from your channel, do:
/mode #channel +b n:webirc