I would say that there are a couple of good options.
add each enemy you spawn to a list, and when you want to destroy them all with your button, go over the list and destroy each.
less good than the first option, imo, but you could use FindObjectsOfType (assuming enemies share a common script that other game objects do not have)
2b) or FindObjectsWithTag.
Unclear as to which part you’re asking about.
I’m suggest that you create a list of type game object
Now, when you spawn an enemy, add it to this list (probably from the spawn script, would make the most sense).
Then, when you delete, go over the list and Destroy the game object.
Note: Destroy(game object) is a unity method.
Do you know how to make a list? Go over it? Add to it?
Lists are part of C#, and used in Unity… they are not a Unity only thing.