Sounds like your low-limit screw on the derailleur is allowing the cage and jockey wheels to move too far toward the centerline of the bike, and colliding with the spokes.
This is bad, because if they hook then your forward momentum can pull the derailleur around in a circle with the wheel, causing significant expensive damage to any of the the derailleur, hanger, frame, wheel, chain, hub and spokes. Plus it leaves the bike unrideable leaving you with a walk home.
The fix is to use a screwdriver to turn the LO limit screw on the rear mech. This stops the parallelogram moving the lower parts over too far.
Downside, is that this might mess with the rest of your shifting - it may have been off-by-one the whole time. Test if you can get the chain to the smallest cog as well.
Another remote possibility is to count the number of Clicks your shifter has. It should be ONE under the count of cogs in your rear cassette. If you have an 8 speed cassette and 8 clicks, then you have a 9 speed shifter and a "ghost" position.