Do you accept old unused paint or know where I can dispose of it in Granbury?

Each location is different. Usually it is the bigger cities that do this. You can local store and ask.

Call them. I'm sure they will be able to get an answer to your question. Some cities and counties have a "cleanup day" that includes things like used paint among other things. Call the city office or county office to see when the next cleanup day for Parker County is.