You can protect a basic program by saving it with
save"filename",p
You can still run" a protected basic file, but can't load" and list it.
That is, after LOADing, the program is automatically deleted when returning to the BASIC prompt. Same applies when stopping a RUNning program via ESC key.
CPC464 Workaround
There is a way to unlock these basic files. This only works on a CPC 464 (real machine or emulator). The used addresses are 464-only (AC01h..AC02h=BASIC RAM, C064h..C090h=BASIC ROM).
Type this:
poke &ac02,&90 poke &ac03,&c0 poke &ac01,&c3
then load" the protected basic file. Done.
Now you can list this program.
CPC664/6128 Workaround
Below installs a resident Fast Ticker handler at BE00h. It resets the "protected file flag" once every 300Hz.
10 FOR i=&BE00 TO &BE0F: READ a$: POKE i,VAL("&"+a$): NEXT
20 CALL &BE00: NEW
30 DATA 21,10,be,11,0b,be,06,81,c3,e0,bc,af,32,2c,ae,c9
In the 664/6128/Plus, that flag is at AE2Ch. For the 464, it would need to be changed to AE45h, or, use the simplier POKE AC0Xh method on CPC464.