You can't. The person who blocked you can only unblock you.
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You can stop someone by sending message only by blocking that person from your Facebook. Messages will only be stopped when you block the person who is stalking.
If someone is sending you messages on Facebook and you cannot reply, the reason is because the person has that certain settings on their account so that only people on their friends' list can message them.
how can i find deleted messages on facebook
If you cannot reply back to someone on Facebook, check to see if you have them blocked. You may also need to change your settings to include them in who can see your posts.
I'm not sure what you mean by that. Do you mean theer sending you a text message if so try sending Stop to the number that should stop them sending messages to you.