Disable Modul/Lightbox Form on Website

Yes, and I think this was discussed in another thread very recently.

Enable Known Visitor HTML ("If Known Visitor, show Custom HTML" in Form Editor » Setup) and then have your KV HTML just contain a single hidden tag, like

<input type="hidden" name="knownVisitor" value="true">

Then, in a MktoForms2.whenReady listener, check to see if that hidden field exists in the <form>. If it currently exists, don't show the modal. Otherwise, initialize the modal. Note you can actually use any element, it doesn't have to be an <input>, just something identifiable and unique.