How can I be open for collaborations, while avoiding interference and keeping the lead?
The two here are not mutually exclusive, nor should they be. Good leaders will encourage any collaboration and input from elsewhere, so long as that is done in a respectful and sensible fashion.
Where this can easily often go wrong is if:
- Alice doesn't really want to collaborate, so doesn't offer much insight into the project to others, or any structure for collaborating;
- Bob does want to collaborate, and sees this lack of insight as a reason to pipe up with lots of his great ideas, dropping by ad-hoc to present these ideas;
- Alice gets annoyed that Bob's ideas are mostly irrelevant or not focused on the task at hand, and are wasting her time, so gets frustrated and offers even less insight into what's going on to try to ward off Bob;
- Cycle repeats.
If you want to break that cycle, then offer detailed insight into your project & its current status, and a structure for the two of you to sit down and mull his ideas (and yours) over. That can take the form of a meeting a week where you present the current state of the project, design decisions, challenges, etc., a couple of meetings a week (maybe one where you present the status, and then another a few days later where you discuss ideas), or something else entirely.
If he's still going way off-piste and suggesting things that are particularly irrelevant or outlandish after that, and this is a recurring habit, then at least at that point you have the paper trail to prove that collaboration with this individual just isn't panning out.