It all depends upon the amount and size of the polyurethane droplets.
If it's two pack polyurethane, then it's a very good paint and will be very hard to remove.
Now IF you've kept your plexiglass well polished with a good wax polish to prevent flysquash sticking and rain to run off easily, it will be much easier as the paint will not have chemically bonded with the screens.
Take a brand new old fshioned razor blade, or large craft knife blade and plenty of soapy water.
VERY GENTLY ease the blade along at a very shallow angle to cut off the paint mist and blobs WITHOUT scratching the plexiglass.
The soapy water will help to avoid scratching.
Wipe away the paint debris regularly with a wet sponge to prevent further scratching and keep the surface wet.
This may take you many hours and try your patience...............don't rush the job unless you really want to buy a new windshield.
Finally, any small traces of paint can be polished out using a proprietry brand of Perspex / Plexiglass cleaner..
Note: If you already had areas of severe scratching before the overspray, then this will probably remain in the scratches.
Nice and gently does it,