I'm not too sure what DLL your using, if it's the nickLUST dll try this:

Code:
alias <marked alias> {
  if ($1 == rclick) {
    return TRUE
  }
}


Change <marked alias> to the alias you chose in call where you marked the channel.

From the nickLUST help file:

Since we return FALSE in the rclick event nickLUST wont call mirc's default popups. If we were to return TRUE or return nothing at all then the default popups will be opened.