This is a horrible situation, made worse by the co-workers involvement.
When you made the complaint, do you know what they said to him? Did they tell him to leave you alone, not to speak to you or follow you or engage you in any way? I hope they did. And apparently he is not following that protocol. You may need to speak to your supervisor again. Or... do you know anyone in law enforcement who could (wearing a uniform) talk to him and just tell this guy not to talk to you again.
Just because someone is mentally challenged is not a good enough reason for others to dismiss his actions or consider him harmless. There is still something in him that has understanding what he's doing.
You are obviously having a fear reaction. I'm wondering after having been stalked for three years if you have PTSD? I was only stalked for a few months and I ended up with PTSD.
You need allies in this. What about the one lab tech who told him to leave you alone? Would it be within reason at all to let that person know of your history with being stalked for years and having to move? If that person could feed that to the rest of the group, you might gain some sympathy and they may have understanding of why unwanted attention is distressing to you. On the other hand, is this a group power play? With the whole group in on it, is it some sort of game they find amusing? And THAT may be the power play.
Make friends with others as you can. Take care. It will get better. Focus on the better part. I have some techniques for feeding your own power. If you're interested, contact me.