Hello again.
Unfortunately (or perhaps fortunately!) this is not a problem that I have ever had to deal with. Having said that, there are various methods detailed online and it all depends on how much you want to risk damaging your camera with the use of tools.
Ideally, this would be a job for a camera repair shop but aside from this, you could try using the rubber sole of a shoe as detailed in this method:
http://www.petapixel.com/2011/11/23/use-a-shoe-to-remove-stuck-lens-filters/
This may sound a little crazy, but you never know! Apart from this, if you have a small enough pair of pliers, you could always try to break or warp the plastic outer ring in a few places to see if you can loosen it. This may be a little dangerous though as you could accidentally damage the lens beyond the filter.
If anyone else has ever experienced this then please let us know!
Thanks,
Simon