drawings on the wall

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I've a 33 months old son and whenever he gets a pen or crayon he keeps drawing on the wall when i am not there.I've tried to explain to him but in vain.When ever he sees me he tries to explain about his drawings like " mummy see my beautiful aeroplane" .I can't appreciate his drawing but only get worried about the markings in the wall with permanent markers,pens and crayons.Is there any way to remove permanent marker drawings from wall? I've tried with clorox oxymagic.But its still there.How can i train him so that he never draws on the wall.Time outs and explaining didn't work.I've even made him clean the wall with his daddy.Any idea from experienced moms?