I promise you that Ad manager and these other scripts are not part of mirc. You either loaded them into mirc or installed something that included the mirc.exe inside it as if this was all part of mirc. (It's against the rules for someone to package mirc.exe as part of their installer package without permission, which i don't remember ever being given)
The fact that the display showd the 1500s delay without showing "time(s)" next to it means this timer executes only once. So the fact that it's repeating means that there's something resurrecting it. Either something inside the 'xtra.ad' alias is relaunching the timer and/or it's checking some other kind of event like CONNECT or JOIN or TEXT to make sure it stays alive.
Since the timer is named xtra.info, you could halt that timer with
but it will likely start up again based on whatever triggered it to start up in the first place.
You don't need to repeat the OFF command in each channel. There can only be 1 timer of the same name, so while each timer can restrict their activity to 1-network or 1-channel, you can't have several copies of the same timer 'name' running at the same time.
Surely such a script came with some documentation to show you how to use it. Sometimes scripts can use a website to keep their documentation, and as they get older the domain lapses and you find the docs replaced by spam.
As mentioned here
the /help from pressing F1 can give you more info about various mirc commands, and the wikichip link can have additional info and examples. If there's a command you're using which isn't mentioned in either, there's a good chance it's actually an alias that's part of a script you've loaded.