The Gimp software has a descreen option. You can find it under Filters → Enhance → Descreen.
You can also do manual descreening with Fourier transformation if you’re not satisfied.
Do Filters → General → FFT Forward then use circle tool with neutral grey color (808080ff) and remove all pattern (anything that doesn’t look like noise except on the main axis (except further on the outside).
Then do Filters → General → FFT Inverse. Voilà!
Make sure you remove the layer transparency first though and work with 600 dpi, then downsize after you’re done with everything.