You don't have to disable the higher resolutions to have it choose 800x600 by default. All that matters is that 800x600 is the first in the list of available resolutions.
Incidentally, you know that you can increase the font size, instead of decreasing the resolution? I'm in much the same boat as you are, in that I found the fonts were far too small in Firefox. So I went into Firefox's setup, and set the minimum font size to 14point, and the problem was solved. Likewise, you're sacrificing an awful lot of workspace on your desktop by decreasing the resolution, when I think the real solution you're looking for is to increase the font size. In KDE's control centre, it's under Appearance > Fonts.
The only reason I can see where you actually need to decrease the resolution, or disable a higher resolution is if your monitor doesn't support the higher resolution. Or more importantly, if your monitor doesn't support a decent refresh rate at that resolution. My monitor will go up to 1920x1440, but I keep it at 1600x1200, because it's only 75Hz at 1920x1440, and refresh rates below 80Hz give me a headache.