Is there a way to hide multiple loot systems so that only the one system or current system is showing at a time?

We now have 2 loot systems as for the new Cata expansion.. With wrath dkp system still showing but there are only 3 options to do something to it... delete it, decay it, or merge it.

I kinda wanted to keep the old system around for nostalgia reasons if it was possible..?

Yup. If you head to Admin > DKP > Loot Systems, you can click "Edit" next to the old loot system, and check the box for "Hide this loot system"