NotThatPortal

This addon will detect any players in your party/raid/instance that cast a portal spell and if the portal they cast is one of the portals you have set in the addon as 'undesirable' it will warn other players in your group not to click it.

You can customise which portals you consider 'undesireable' in the options.

You can also configure where and what the warning will look like:

 If multiple undesirable casts are detected the addon will throttle warnings to prevent spam.