how can I put ribbon around a butter cream cake and not have it absorbe the moisture?

If you're using a crusting BC, allow it to crust. When there is no moisture on the surface of the cake, there is nothing to soak into the ribbon.

Another method is to totally pre-coat the ribbon with crisco and uniformly "soak" it. Test this first. If you do this on navy blue ribbon, the ribbon will turn black.

You can also line the back of the ribbon with packaging tape.

You can wrap the ribbon in contact paper before attaching it to the cake

thank you for the suggestions.

I only use fondant ribbon. No "soaking" problems at all!

Here's one of mine, delivered today, with fondant ribbon.