Torching it would be the best option, IMO.

I have done this to several frames in the past. My first project involved a hack saw to remove the bulk of the TT cable guides, DT shifter mounts, and derailluer hanger. This was followed by a bastard file, smooth cut file, and then sand paper. It came out very well, but it was a lot of work, and I have the impression that I may have taken some material off of the frame tubes as well.

Second project involved my Metabo 4 1/2" angle grinder (something like 12,000 rpm). Worked like a charm for the first two guides, then cut right into my toptube. Not recommended. That frame has been disassembled and is now a pile of tubes and lugs to serve as another project.

Third project involved my Oxy-Acetylene torch. By far the best and most effecient. I heated up the braze ons till they were cherry and grabbed them with a set of pliers. Off they come, no problem. Then reheat the area and use a fine metal brush to get all the extra brass off the spot while its hot. No cuts, dents, or residual damage of any kind. This will only work with brazed fittings though. If they are welded you will just heat the things up until they distort the tubing. (not good)

Project four: was gonna torch off the braze-ons, etc. but someone decided that they needed my torch more than I did. Alas, I think the fittings will just stay as this is the cheapo beater bike.

-Pete