Everyone has good ideas (for the most part) but here are the steps I would take.
1) Tell your barn manager. They need to know because this could be happening to other horses. They need to be aware if you are going to do some investigating :)
2) Set up security cameras. Make sure there is one facing towards his stall, and then one that is facing towards the barn aisle to catch the face of the person and the act of overfeeding being done.
3) If the person is a boarder or someone who frequents the barn, keep reading. If not, skip to step four. Confront the person calmly and politely. Tell them you are flattered that they are looking out for your horses well-being, but tell them why the signs are up. Often, people who admire horses but don't know much about them don't understand the careful nutrition horses need. Tell them how alfalfa, due to its high nutritional content, can harm a horse if not fed properly. Explain what is going on with your horse and maybe even show the person what the vet has said. Tell them that feeding the horse irregularly and too much can cause colic, and if they really cared, they wouldn't want the horse to get hurt.
*Most importantly, don't tell the person you set up cameras so that you can monitor them after you confront them. If they deny it, then you can pull out some tapes.*
3b) If the person complies and understands, monitor on video cameras to make sure extra feed still isn't being given. If person continues, read step 4.
4) File a police report. Be prepared to back this up with some video evidence to make the case stronger. Horses are valuable and fragile, and overfeeding in some situations can be counted as animal cruelty. As a minimum, this can be classified as trespassing.
I hope this helps and good luck! I have always admired how cute Fjords are :)