In the past, I took a dremel tool with cutting wheel and cut first a horizontal slot onto the head and used a flat screwdriver to see if I could unscrew the thing. And in the cases where it is indeed riveted, a little more cutting and the head comes off and you can remove the cable guide. If you're careful, you can try to avoid damaging the guide so you can reuse it. But be prepared to tap the hole in the BB shell to put some threads. You'll need to upsize the screw as well and also find a short one or take an existing screw and cut it short and rechase the threads if possible. To prevent the screw from falling out, try some loc-tite compound on the screw. If the cable guide is damaged, I think a number of places online sell those and your LBS may carry them as well.