Help!! Delete Multiple Test Messages

I can't alt- click on multiple messages and delete anymore. I get hundreds of texts from simplisafe, petsmart, and many other places a week. I can't sit and delete each one by clicking X , wait window pops up to ask for confirmation then clicking yes. I can't possibly do this....this takes too much time. This is not acceptable. How can I have this expensive piece of equipment and i can't even streamline deleting hundreds of text messages! I will have to stop using texts on my iMac. Im using Mojave OS and I want to go back to Sierra!