Having a dog neutered is a good thing and can help with the dog marking it's territory as they will not mark their territory as often as before.
However even when the dog has been neutered they can and still do sometimes mark their territory so having a dog neutered is not a complete cure for a dog marking their territory.
Neutering your dog does help curb the behavior of the dog marking it's territory but dogs still sometimes mark their territory even when neutered.
You can stop the dog from marking it's territory in the house or wherever by soaking the area with pet urine enzyme cleaner.
The dog will then stop marking in those spots.