I guess that it might be an idea to limit the SPROCs that can be called. It might be an idea to make it impossible to activate any SPROC that is a system SPROC. That would require screening of the $SPROC variable.

Whitelist. Make a list of all the acceptable values of $SPROC, and don't execute anything that's not on the list. In information security, it is always far, far better to use a whitelist than a blacklist. Anything not expressly allowed is automatically verboten. Don't put anything on the list that doesn't actually need to be there.

Plus do what bart suggests above, with the placeholders. I was going to say that, but he beat me to it. It's good advice.