Hello Sue and thanks for stopping by planDisney with such a great question!
My own 
MagicBand collection is pretty impressive, and I do have some favourites that I wish I could deactivate in an effort to save their battery life.  But sadly, to the very best of my knowledge, that's not something I believe is currently possible.  
However, as I'm sure you already know 
still functioning MagicBands from previous visits can be reused when returning to Walt Disney World Resort, but you'll need to bear in mind the 
older MagicBands may have difficulty in receiving Disney PhotoPass Attraction videos, or that some Attraction photos may be limited.
Thankfully MagicBands are here to stay and can be purchased in 
almost any merchandise store at Walt Disney World Resort or from 
shopDisney.  But have you hEard about the all-new 
MagicMobile that's coming soon to Walt Disney World Resort?  This practically perfect in every way sounding enhancement to 
My Disney Experience will enable Guests to harness the technology already found in Apple Watches, iPhones, and other smart devices when visiting Walt Disney World Resort - it really will work just like magic! 
Sue, I hope I've been able to provide a little insight for you today.  If you ever have any more questions about visiting Walt Disney World Resort, then please do pop back to 
planDisney!  We're always happy to help in any way we can.
Have a great Disney day ~ Jane