The SX-1 has a jumper to disable memory, yes. But you would have to open it up, and you might as well take the simm out instead of fiddling with the jumpers. The SX-1 also has a switch on the back that disables the harddrive and the floppy drive (but not the memory! go-figure).
Personally, I prefer the cd32 + sx1 to an unmodified a1200.
Can run os3.9
cd32 and cdtv compatibility
boot from audio cds
can use any AT keyboard
can boot vcds with mpeg decoder (haven't tested this as the mpeg decoder cards are rare and expensive)
can't disable fast mem
rgb cable bumps into power cable
no upgrade options
The promodule and sx32 are much better upgrade solutions for the cd32, but they cost way too much. I would reccommend an a1200 if you can't find a good price for cd32 expansion. However, if you know someone that is good at making circuit boards and do-it yourself electronics, you could always make your own floppy controller