How can I reclaim quick-keys from special characters I never use?
Is the ⌥⇧P shortcut a default in Byword? In OS X, shortcuts involving option without command or control as a modifier are for inserting special characters, like the symbol you're seeing inserted. I'm surprised that you're only seeing this functionality recently, as it's been there for many years - perhaps a recent Byword update changed something?

To my knowledge, there's no way of disabling these in System Preferences. You might be able to create a custom keyboard layout that would remove these, but I haven't tested that. I would suggest either setting a different shortcut for the functionality you want in System Prefs. The other (more awkward) way is to take advantage of the fact that the special character shortcuts only do anything in text entry fields. If you don't have a text entry field active, the shortcut should return to it's previous functionality.