How do you stop a neutered dog from marking?

Supervision, and prevention are the answers!

When you can't be watching the dog like a hawk, or you leave the house, your dog needs to be contained to either a dog crate (if it's crate trained) or part of a room with the help of a child-gate or two. Your dog should not be having the run of your home until the marking problem is under control. Only when you catching him "in the act" should you reprimand him. It's useless to punish a dog for something that happened previously, and it can lead to other behavioral issues.

A shaker can can be made by taking an empty soda can and filling it with a few coins and then taping the opening closed to prevent the coins from flying out. The shaker can makes a lot of noise when you shake it up and down. Watch your dog for signs that he is going to mark (for example, he's doing circles around his "mark"). As he is about to lift his leg: shake the can only once to get his attention. The loud noise should frighten him and interrupt what he is doing, firmly tell him "NO!", and take him outside to relieve himself.

Clean the areas where your dog has marked with an enzymatic cleaner, such as Nature's Miracle, or Simple Solution. These cleans are very effective at removing the scents which might be attracting your dog back to mark.

Establish a clear hierarchy! To reduce marking behavior, it is very important that your "pack" has a clear hierarchy, and that you are leader of the pack! Simple steps to establish yourself as pack leader include not allowing your dog to sleep on the bed with you. Sleeping on the bed elevates him to your status and sometimes creates status confusion. Once your dog understands that he is not the leader, he'll feel less need to mark. He can rest; that's someone else's job!

A "Belly Band" is a training aid you can try. It's wraps around the dog's belly and prevent him from urinating on objects. If the dog is wearing the belly band and he marks, the belly band will function like a diaper and hold the urine. Dogs don't like being wet and they will normally stop marking once they learn that marking makes them uncomfortable. Try doing a google search for "Belly Bands" to see these products. Belly Bands can be a great tool to help stop marking, but they shouldn't be worn all the time.

If nothing seems to help the marking problem, including containing your dog when it can't be watched, then having a Vet exam is a good idea. Conditions such as a urinary tract infection can cause a dog to have accidents, that it can't control.