The ATF's usual stance is "once a full auto, always a full auto." Doesn't matter if you pull the parts and weld the receiver to prevent it, it's still a full auto. About the only way around it is a full de-milling.
If a firearm is a transferrable FA, I don't know why you'd nuke the investment by doing this. If it isn't transferrable, you're going to need to either render it so it isn't a firearm anymore or swap enough parts (incl. the receiver) so that it is a "new" semiauto built off a parts kit.