I am converting a 7-speed to a single speed. The gear cable guides (?) are not welded to the frame, it looks like a brazing or solder job. It's an aluminium frame; can I remove these easily by heating? It's an early 90's Giant cross frame.
They may be glued on or spot welded. Aluminum usually isn't really brazed or soldered. In either case you stand a good chance of damaging the frame if you use heat to remove them.
A Dremel tool with a cut off wheel or grind stone followed by smoothing with a fine file will work if you are very careful to only cut the fitting, not the frame tubing.