How to boil away water from a sauce without it splashing?
If, when reducing a sauce, you are getting splashing outside the pot, your pot may be too small for the application.  Use a larger pot:  a wider base will allow more surface area for evaporation, and higher sides will make it harder for the content to get out.

You can also purchase a splatter guard, which is like a flat sieve or mesh that you can place over the top of the pot to reduce splatter without blocking air flow



although these are more typically used to reduce the splatter from frying.