Replying to Shaking off someone who wants to hang out with you...

Coming from an adult this may seem like such a juvenile question....but...how do you typically shake someone? My brother briefly dated a girl who I became friends with during that time-- we also became facebook friends-- they broke up after about 6 weeks of dating (about a year ago), and ever since she messages me all the time to try and get together. I usually find some way to decline but its getting more and more awkward. Like today she messaged me and said we should get together soon, and I said "yeah maybe grab a mocktail when the weather warms up" (I'm pregnant so no cocktails)-- and she always wants to get something concrete so she is like "well what are you doing this weekend and next weekend?" I really hardly have time for my actual friends-- let alone this gal, so what should I do to shake her? If I get nothing out of this thread but hilarity that is fine with me, too.