The Webform module has a feature now where you can mark a webform as open/closed to new submissions.

Webform Alternate UI is hiding that interface, at first because there was an array key naming conflict that made it incompatible with this module, but I think that's since been resolved.

So we just need to figure out where to expose it in this module's UI. The natural place where it appears (the "Submission settings" fieldset under the "Form settings" vertical tab on the node form) seems like the right one to me.