If the brass insert is spinning you can probably pull it out of the plastic on the snorkel. Take the screen out then gently pry the two apart, worked for me. Once you have it apart, mole grips on the brass insert, and undo it.
Alternative I have used before as well, is to drill a small (2mm) hole in the plastic through the brass insert until you have gone about 2mm into the brass. Then take the drill bit out of the drill, turn it round and put the plain end (bit that normally goes in the drill chuck) in the hole you just drilled. Be careful not to overheat the plastic and melt it when drilling, you need a good sharp drill bit. This will anchor the brass insert and allow you to undo the screw. Once the screw is out you can screw a short self tapping screw into the hole you drilled to anchor the brass insert permanently. I also put some super glue in the hole first to help stick the insert in place. Worth running a tap through the threads to clean them out at the same time.
2001 RC51 SP1 (modified)
2007 GSXR1000 K6
1994 CBR600 FR (very modified)