mIRC displays its own default text for various types of IRC Server events, such as users joining or parting a channel, however you can modify or suppress this by using a script.
The ^ event prefix
You can prevent the default text for an event from being shown by using the ^ prefix in an event definition. This allows you to show your own custom event messages.
on ^1:JOIN:#:echo $chan Joins: $nick | halt
This line is triggered by a JOIN event and shows your own custom join event message, /halt prevents the normal message from being shown.
The ^ events do not replace your existing events; your normal events are independent and are still processed whether there is a ^ event in a script or not.
If you only want to halt the default text without /halting the entire script, you can use the /haltdef command.
You can check if a script has already halted the default text by using the $halted identifier; it returns $true if a user has used /halt or /haltdef in a ^ event, and $false if not.
The ^ event prefix works only on certain events, such as ACTION, TEXT, JOIN, and so on. You would need to try out the ^ prefix with an event to see if it works with it.
Note: Halting the default text for an event affects how mIRC displays the most basic information about IRC events to a user, so it should be used carefully.