It's not impossible but it does take a little effort. Here's how I did it while I was away in Africa - expect it to take 20-30 mins.
1. First check your lens very thoroughly. If it shows signs of some elements being decentered, the IS behaving strangely, zoom stiffness or focus issues, you should send it back to mummy. Also, if the filter is a B+W or another in which the glass is removed from the lens side, I recommend you send it to Canon...
If it's not a B+W filter (and you're feeling brave), you can proceed... If it is a B+W and you're still feeling brave, you can proceed too but you are going to break the filter glass which is hazardous for you and your lens.
2. First you need to remove the filter glass from the structure. If the filter is secured with a clip, you can pry the clip out and remove the glass. If it's secured using a threaded ring, get a lens wrench. These are available from a lot of places - I use this one http://www.ebay.com/itm/Spanner-Wrench-3-Tips-For-Camera-DSLR-DC-Lens-Repair-Repairing-Open-Tools-DC313-/190835498564
. This should let you unscrew the ring without damaging your lens.
3. It will help if you rig a vacuum cleaner to suck all the filings away as you work.
NB - I do not recommend a Dremel for this job.
Cover the objective with something to prevent it getting scratched - you could trim an EZ Off but be careful grit does not get under it. Next, try Spooky's suggestion cutting slots. This is best done with a needle file. You get one that is about a millimetre thick and 5 wide, with a square edge - something like the fourth file in this image http://upload.wikimedia.org/wikipedia/commons/thumb/9/9f/NeedleFiles.jpg/200px-NeedleFiles.jpg
If this works and you can unscrew the ring, great. If not, go to step 4...
4. Continue filing the slots until you are just about to break through the lower surface to the lens' filter mounting thread. This takes some patience but it can be done. Now make another slot about 1 cm away from one of the others. This needs to be just as deep. Again, you need to be really careful that you don't scratch the front surface of the objective or get crud into the mechanism.
5. Once part 4 is done, pick up a decent size pair of pliers and gently wiggle the thin section back and forth. This is where the filter glass would break - you need to get it all away from the lens and then install some protection. The idea is to fatigue the thin section that remains where you could not get to it with the file. Eventually, the filter ring will snap. Once it does, you can carefully lift out the filter ring.
6. Vacuum the entire lens, your self, your bench and tools before you attempt to blow the grit off. This minimises the amount of crud you will spread around with compressed air.