I would be direct. I have found that if I am anything less than totally blunt with clingy people, then things just get worse and worse. And any impulse saying that it'd be rude or wrong of me to speak up just ensures the clinging will never end. I am just getting out of a destructive situation where if I had said something at the start the whole mess would never have happened. If the person has a condition that can affect social boundaries then they doubly need to be told. You don't have to be nasty but if you aren't firm at the start it just gets worse and worse.
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams