Wrongly sent texts on iMessages

First, when you open iMessage, open the conversation, then tap on the icon at the top of the screen, then the "i" button labeled as info, click on the phone number/name, the scroll to the bottom and tap the button that says "block this caller".

you cannot stop them... unless it's from consistent phone numbers, then you could block the number(s).

You can turn on a switch in Settings > Messages called 'Filter Unknown Senders'. This will make it so that texts from numbers not in your contact lists won't produce a notification and you can also sort them out in Messages.