The context is important so I will start with that. A group of organizations decided they wanted a specific project done and to hire a full-time person to do it. They all contributed money, but the job is officially with one member organization. I was hired for the job and deal with all of the organizations in some way as I work through the project.
There is this one person I must deal with who I find to be a problem. They work for one of the member organizations and are their employer's main (but not the only) representative on the project. The project is a small part of their job, maybe one or two hours a month, but it is 100% of my job. They seem to be pretty chummy with my direct boss so I need to be on reasonably good terms with them.
But they are a real piece of boundary-crossing work. They say inappropriate things in work meetings, they keep angling to get my personal contact details (which I do NOT trust them not to abuse), and generally act too familiar.
The major problem though is that they keep asking for help with their own work, things that have nothing to do with the project I'm working on. One time I caught them lying about why they needed my help - it came out when I asked for some clarification on some things - which really turned me off. Also their tone has changed pretty quickly from asking for help to telling me what to do.
Currently I'm trying the tactic of being a little less available and giving "here's how to help yourself" type of advice. (The last request for help was for work that was not only already done, it was done by their own department! I don't even work there!) I really do have a lot of work to do on the project and a big deadline looming so I will definitely say that next time I get a request, but does anyone have any other advice?