In flash say I want to eliminate everything a function does.
I know I could nop it all out, and that has worked for me.
But to save time could I just replace the 1st part of it with a return void?
If so is there any way the code after the return could ever run?

return is always halting, so provided the return is not the antecedent of a conditional branch - ie the return can always be reached - , then no, there is no condition under which successive instructions will be processed.

ie if you replace the beginning of the code say
d0 30 24 00
with
d0 30 47 00

then nothing after the 47 (return void) will execute.

note: make sure you match your return types.

You can wipe out the d0 30; as you're just immediately returning from the function, which'll reset the stack; it matters not whether the parameters are valid.

This is one of my favourite ways of doing invulnerability hacks.