Is there an app or any other way to auto-delete specific contacts/numbers' text messages when received?
Currently you don't need an external app for this, it has been a feature of Android since long time. Messages from blocked senders are deleted upon arrival so they don't even appear on your device.
Go to Phone -> More -> Settings -> Call blocking -> Block list and add the number you want to block.
Alternatively, go to your phone log or SMS list and select More -> Block number.
When you block a number, it automatically blocks all incoming calls and messages from that number and also deletes all of the conversations you have had with them. This is so they won't be able to reach you at all and no matter what they do. they won't be able to contact you.
When you block a person, it also erases their phone number. If you try to call or text them, it also won't go through since you have already blocked them. There will be no problem since neither of you can contact each other. It is the best way to go.
You can just download an app from Play store - it can block text messages and even phone calls.
Try Truecaller, add the contact to the Blacklist which will lead to the block.