What to do with untradeable TF2 scrap?

Craft something with it (even a stupid weapon)

Yes,you can craft weapons untradeable or combined with rec and you have a untradable rec ^^

You can use it with an untradable unusual haunted scrap to make the HHH axe.

You can try to create some of the untradeable weapons or hats as described on the wiki.

delete it.

You can combine it with more scrap to create untradable refined metal, which is the first step toward making a new untradable item.

Don't worry - you just set yourself back half a piece of scrap, which isn't so bad.