How to tell people no?

I've seen this discussed before and never thought it would really apply to me. How do you tell someone that they aren't invited? Our wedding is still a year away and the guest list hasn't been finalized yet, but someone I knew in college (not a close friend) asked me "hey babe am I invited to your wedding?? 😍😍😍" This came to me as a Snapchat message after she saw me open an early birthday present on Snapchat (bride and groom luggage tags from one of my close friends).

I guess I never thought I would have to deal with this because I didn't think anyone would ask so candidly... My fiance and I aren't very social and our friend group isn't large, and he has a huge family that we have to invite, so we can't accommodate extra guests. I'm not close to this girl at all, and she was kind of like this in college too, asking to do things because she knew people wouldn't say no. The last time I even saw/spoke to her was in 2015.

I know I'm probably just being a huge baby and I should just say "no", but how do I word that? My inability to turn people down is a huge character flaw of mine and I can see how wedding planning is going to be awful if I don't grow a spine.