Load of crapinsky guys. You see them mark, growl straight away and make him go outside. When he does something, praise him. Then he can come back in. You have to keep telling him no (sturn voice) when he does something, no smacking as you will confuse him. He will learn its not on in the house. Its bullshit that a dog needs to be desexed, sorry this shits me to tears. I have 6 stud dogs in my home and none of them mark. Its a matter of keeping at them, cant be lazy, just keep putting them outside. If you are busy doing housework, crate them. Its easy really.
If there where reps here you would be getting some...
Marking is a way dogs have of showing the pecking order, who is Alpha and who follows. The dominance is some thing they decide in play and every day activities, although you may see playing dogs as playing, just like kids, they are deciding who is boss around here...
Marking is a reminder of who is boss. Unfortunately your Alpha male might be forgetting he is a member of a human pack as well, and in that family, you are the Alpha. It is important to show him this is not acceptable in your home.
The difference between marking and nervous drop or regular peeing is not easel seen by us humans unless you are very attentive. Dogs that make, rarely need to make more then an ounce are the time, so they can go around a long long time and mark. It is generally done in a perimeter and shows where their domain is.
Very important you show your Alpha male that His domain is actually yours, and because you can not go around marking your home, you have to become More creative about it. Don't be mean to the dog, they will respond with understanding that being mean is OK. As Sivaro said, be stern and fair. Given the chance, dogs just like humans will take the chance to repair what they did wrong.
If you have a yard, bring them outside, Let the Alpha male mark the perimeter of the yard or area where he is allowed to be, this will satisfy him. Within the house, use a spray bottle to cover his markings as soon as you see it and correct him.
Example, you see the dog mark the wall, Shout NO! use a bottle of Cinnamon water* and cover the area, Your god will go to smell the new scent. Do not clean it up right then, take the dog out to the closed in pen so he can not see what you are doing, then go inside and clean up. Once you have the area cleaned up, Use the Cinnamon water again to mark the area, spritz a few times and let him back Him. The dog will naturally go to smell that area to see who has it marked. When he does, look at him like you mean to say "Now I am the boss!" When your dog tries to roughhouse with you by play, make sure you take dominance over him vocally and physically.
This is not always some thing a younger person can do, so it may be best left to a parent or older brother. Mind you, I have seen large German Shepard's who completely submitted to a young child. Emotions play a large role in this.
The actual smell matters little, it is an identifier. Each dog has his own smell, what is important is to cover the smell with "Yours" and to have him recognize it as such.
To make cinnamon water, take a few cinnamon sticks, (2 or 3) boil them in water, then take the sticks and water and put them in a spritz bottle. The smell is very agreeable to most humans and very marking to animals, it is almost like a pepper.
PS: Cinnamon water is also a very good ant and spider repellent, most small bugs will feel like they are walking on fire when they walk over Cinnamon so they stay away from it. 
