General rules for online games.

Free experience is a double-edged sword – it exists, but ultimately leads to equality.

It introduces a useful tactical element because it can be summoned through the astrologer.

And that's a very good plus. It increases the variety of the game.
The choice is this: either allow joining, or allow the creatures to exist.
If we don't want joining, we remove it.
If we want the creatures, we leave it.
Until there's a mandatory stream, you won't be able to track whether your opponent has recruited neutral creatures from these weeks or not.
In the end, you could try banning only the native weeks for both players. The chance of joining decreases, and it's not very profitable to keep non-native creatures in your army.

Added 9 minutes later

The bottom line: the absurdity and contradiction of your two messages.

-In the map properties, you can prevent creature weeks from appearing (ban them!).
-No, that clearly won't work (allow them!).
-But why have creature weeks if you're not going to use them? (You don't want to use joining, which is inseparable from the week, so by banning joining for these creatures, you can only choose not to use the week, so why allow the weeks in the first place?). The fact that the statement contains several logical conclusions at once led to your incorrect judgment. Let's dig deeper ;)