They would still be able to contact you, so if you want to make sure they don’t, you’d need to block them:

However, deleting the conversation doesn’t necessarily remove a contact who is not in your list of Facebook friends.

Keep in mind, the other person will still have their copy if the hadn’t deleted it as well.

Click “Delete”, to permanently delete your copy of the conversation.

Click “Archive”, to archive the conversation so you won’t see it but can access it later.

Right click, or tap and hold on mobile, to open the submenu for a specific conversation.

The first step you can take is to simply delete your conversation with them. So, here’s what you can do to remove them from your Messenger app. Or you might have received annoying messages from someone that you don’t know, and you’d like to stop this person from contacting you. However, it is not uncommon to be approached by people that you don’t know, for example, if you’re selling something on Facebook’s marketplace app someone interested could message you to receive more information on the item you’re offering. We are all used to using the Messenger app to talk to our friends and it has even become more common to message them there than to ring them or send them a text. Can I delete someone from Facebook Messenger?

How to delete conversation in messenger