There are some answers here that are very close but the exact meaning of
"beating a dead horse" is that the subject under discussion has been "cussed and discussed"
(we have said it all!) and
there is nothing new left to say.
It is generally used when a person wants to continue talking about something after all
reasonable discussion has taken place.
Ex.:
"We have already exhausted the subject so we are beating a dead horse."
"Talking to the wall" means you are talking to a stubborn person; somebody who has a closed mind on the subject, that is, a person who has made up his mind, and will not consider any further possibilities.
Ex.: "Trying to reason with him is like talking to the wall."
Both of these expressions suggest that you are "wasting your time" on a subject but they are NOT synonymous.
vicente