My LG G2 will not allow me to delete multiple messages, I've tried everything to get rid of old messages from a long messaging thread, I've tried a couple apps, doing it by hand etc. but nothing works, they just won't delete. Since the thread is so long texting is almost impossible, it is slow, the keyboard goes slow, pulling up the thread takes forever and sometimes it doesn't even pull up it just says messaging stopped working, however this is only a problem for this particular thread.
If this is your longest thread I can see the issue. Did you try other messaging apps or message-deleting apps? You can set the message limit a lot lower and older ones will be over written. I had this issue a while back and it took a very long time and even froze my phone. I wound up resetting my phone and clearing messages frequently after setting a low limit. There are ways to rid the phone quickly of texts...but its a hidden path and Verizon considers it illegal to do so. They are on your phone though as I'm sure you've noticed before that after changing phones your messages are gone. So they aren't in the cloud.