Just say no
One of the annoying things I used to do often was to say "not sure" when I was invited to social events. I thought I was preventing disappointments but was only delaying and amplifying them.
It's actually super frustrating when you are on the receiving end of "not sure" as an answer to an invitation. All it does is to immediately raise questions like "Do I include you in my plans or do I not?" and "when will you give me a definitely yes or no?"
I was super lucky that a friend at uni (eventually) complained out of frustration, and the moment I suddenly realised that saying "not sure" was actually hurting someone is still very vivid.
So if you know someone who does that, consider pointing that out politely! If you are the one who says "not sure" often, simply saying "no" is still better than "not sure" regardless of how close you are to those inviting you. If you are never sure because you're disorganised or always super busy, don't use that as an excuse to say "not sure" — consider "can I get back to you by [insert date]" and commit to not being disorganised.