Catch Him in the Act When Marking

You´ve cleaned his previous markings, and the markings of other cats. You´ve applied a citrus scented solution to further deter your kitty´s marking behavior.
Now you need to use Corrective Measures when you see him marking something. A water pistol, a loud noise, or a firm ´NO´ when you actually see him marking something is a last resort to stopping this behavior. You must eliminate the ´trigger´ to his marking, or it will do no good to correct him.
Never use Corrective Measures, if you don´t actually catch him in the act of marking. This will not stop the unwanted behavior, because he doesn´t associate the Corrective Measure with the unwanted behavior.
You will find those more tips on Corrective Measures in Answers for Common Problems: General Training and Correcting a Bad Kitty.