Actually, I never received anything that said I completed 50 hours--I just got a free membership that was valid until 5/31/2007.
As Martin said, those volunteer badges that were issued via the orientation with your pictures on them have to be surrendered if OMSI asks for them. While I understand these badges have sentimental value, they do not belong to us. They belong to OMSI. If they ask for them, we need to give them back. It's like a gym card--when you join, you agree to give it back when you leave. It's in the verbiage of the documents you sign when you agree to be a volunteer.
The Star Wars special access badges (black, no picture) are fine to keep.