Welcome to the Forum @anon98498251 and thank you for reaching out! 
I completely understand your point. I’m afraid that as it stands, there isn’t a way to limit guests to be assigned to only certain Projects or tasks.
There is a thread in the #productfeedback category regarding this topic: Limiting what users show up as Assignee options for tasks. If you haven’t yet, I would recommend you to upvote it!
Please let me know if there is anything else I can help you with! Have a great day!