How to TRULY delete messages on iPhone?

I'm running out of space on my iPhone. I go to Settings -> General -> Usage to see what I can delete and it says I have 2.8 GB of Messages! So I deleted tons of my old messages. Huge lists of text from old friends and ex girlfriends are gone now… But the number didn't budge! It's still at 2.8 GB! How do you delete these old messages?! They're not even accessible anymore. I've synced it, I've looked stuff up, I've tried everything. How do you get rid of this wasted space?!

Deleting messages don't really help with memory, try deleting some of your photos or apps

Check your email account.