I have sent a message to someone that shouldn't have got it. They haven't read it so I reported it as spam and deactivated my account. Will this definitely stop the person from seeing the message? Has anyone tried and tested this?
After marking spam or abusive, your messages will not be readable to that person.
When you report a message as spam, that person will not be able to read your messages in his/her inbox, but what that person has sent to you will be readable for him/her in his/her inbox.
No need to deactivate your account.