Hi CAP417, on an android
if you go to settings and then applications you will see application Manger at the top. Open that and scroll down to message+, open that and then look for the "more" at rhe top. You will see a remove update button. After choosing that you should see a verification asking if you intend to go back to the factory install. Once done you should now be able to turn off updating by going to the play store. Look for the three horizontal lines at the top in the search box. Click that and choose my apps. Look for massages+, choose it and on the apps page you will see three dots on the upper right. Touch the the dots and a box with auto update and a check should appear. Touch that check to make it disappear. You now will not update.
I myself did this plus had to switch to the Samsung messaging app as I continued to have issues and in following others as they reported issues I realized if a contact had the update there may be a conflict if I did not. I dont know if that issue still exists and the samsung app is plain vanilla but uses the default picture editor as messages+ did once upon a time.