Except yours doesn't work either... The red part in
on *:dialog:forbiddennicks/channels:sclick:1:
is interpreted as a command because of the slash, so when you click the control with id = 1, the Channel Central dialog pops up.
To fix this, you need to do one of the following:
1. disable the Channel Central dialog through Options
2. alias the /channel command
3. add a space after the slash in the dialog event