If the rooster sits still, the filing could remove the very tip but, to remove more, you'd have to get cutters. I'm not sure which kind of cutters work best. My father has a large workshop so I tried the ones that were the best size and sharp but I couldn't even make a dent in his spurs. The spurs were about half an inch in diameter (maybe more) and four inches long (curled up though). I saved the one that broke off. I hope your rooster is smaller in which case it shouldn't be too hard to trim them.
Here's a photo of the dewclaw that Sugar broke off when his son, Speckles, attacked him, 12/16/06. The entire dewclaw was longer because the part that didn't break off (bloody and alive) was another inch or so long.