Your old client connection is called a ghost

If the network you're on uses nickserv, the standard format to get rid of a ghost is /msg nickserv ghost <nick> <password>

Replace <nick> with the nick that the ghosted connection is using, and <password> with the registered password for that nick.

This can't be done using an ON TEXT event, but can be scripted as an alias or using the ON INPUT event.

Personally, I prefer the alias option for something like this

Code:
alias ghost {
  if !$1 {    .echo -a Usage /ghost <nick> <password>  }
  else {    .msg nickserv ghost $$1-2  }
}

To activate the alias, type /ghost