Summary
The CLEAN function in Excel is a little-known function. As its name suggests, it cleans up text. Or more specifically, it removes all non-printable characters from text.
So wherever you see one of those ugly characters that appear as a square, this is your function. It also removes in-cell carriage returns, which saved me a lot of pain today.
Syntax
=CLEAN(Cell)
Cell is the cell containing the text string you're looking to clean.