How do I professionally communicate to a colleague that I am not interested in helping with his project anymore? I'm a PhD student in a pretty large lab, and have come from a more theoretical background than the other members, so sometimes they ask me to help out with theoretical parts of projects. Generally I like this, but a postdoc recently asked me to help out with his project, and I tried to help but it has gone poorly. We've gotten nowhere, I find his problem ill-defined and don't actually understand what he wants from me, and he even seems to want me to take lead on the project. At this point, I'm not interested. Recently he emailed me asking for progress and I told him I hadn't had time to work on it, because I had 3+ other projects that I'm working on.

Then he suggested emailing my supervisor to ask if my supervisor could "give me more time to work on his project". I don't want this. Also, I briefly talked to my supervisor about this, and he agreed that it wasn't worth allocating time to if I wasn't interested. How do I communicate to this guy that my supervisor doesn't dictate what I spend time on, and I'm just not interested in following up at this point - politely? I literally just don't know what to say.
Your project is nice, but I now decided for myself that I can't invest more time in it. I'm too busy with other projects which have priority for my current research interests (so, talking to my advisor won't fix the issue). 


And just repeat that same basic point when he tries to argue/discuss with you.