Based on my experience, I recommend a software which is named iOS Private Data Eraser, which is only available to iOS users.
iOS Private Data Eraser is a professional data erase tool, you can use it to completely and permanently erase the phone's data, such as Contacts, Messages, Call history, Notes, Photos, Calendar, Reminder, Safari's bookmark. The process is very persistent, you can do it according to the instructions, intuitive steps to make it easy to delete data. This software applies to iPhone 7, iPhone7 Plus, iPhone 6, iPhone 6s and other iOS devices, the current all of iPhone models are compatible with this software. The process is also safe, it will not secretly save and record your phone data.
Steps to Permanently Delete SMS Messages from iPhone
Step 1. Run the program and connect your iPhone to your computer
First you need to download and run dr.fone toolkit on your computer. Then, prepare a USB cable to connect your iPhone to your computer. At the same time, find and click "Private Data Eraser" from the main interface.
Step 2. Scan private data on your iPhone
When the program is aware of your phone, you need to click "Start Scan" to scan the phone's data. Please wait for a while, when the scan is over, the scanned data is displayed in the window.
Step 3. Start erase messages on your iPhone permanently
Next, you can preview the data one by one. From these data and find the message you want to delete. Then, click on the "Erase the Device".
Next, the program will pop up a window, you need to enter "delete" to confirm your operation, and then click "Erase Now".
Please do not disconnect the phone from the computer while deleting message. When this process is over, you will see an "Erase Completed" message on the window of the program.