Hexidecimal Non-Allowed Characters in a Unicode System

We have a function module that we've written to replace non-permitted characters with a space in transfer rules. We see a lot of invisible hexidecimal characters coming in free form text fields. This work great for English. However, we have a Unicode system with other languages installed. We are also getting the hex characters in other character sets.

Has anyone dealt with this issue and if so what was you solution?

Thanks!

Al