iRule to display site down irrespective of the state of pool members

If you are going to take this action unequivocally, then there is no need to associate a pool with the Virtual Server at all, unless, of course, you are going to "turn on" and "turn off" this feature. If that's the way you intend to approach it, however, you might consider leaving the rule as-is, and when you want to active the feature, simply disable all of the pool members.

In any case, if you wish to do this without condition, it is simply:

when HTTP_REQUEST {
    HTTP::respond 503 content { ... }
}