I want to remove the custom tab from the opportunity form but can't seem to do this. Removed the checkbox but it still shows up. Any idea why this might be?

Just hide or move what ever is under custom tab - it will fall off automatically

There's actually nothing selected in the Custom Sublist Fields. This is what is currently showing and I have no idea what it is...

Remove the label, sometimes that does the trick

I just realised - new opportunities do not show the custom tab, but those opps already created do show it. I can live with this. Thanks all.