You have countless strange facebook accounts added to the group chat list with sales ads. You do not want to be bothered by constantly receiving message notifications, but when opened, they are all promotional messages.
To block strangers from adding to the group chat list on facebook you do the same with blocking spam messages on facebook. If you do not know, then you can refer to the following article.
Step 1: On the interface of the Facebook homepage of the account you want to prevent strangers from adding to the group chat list, select the triangle icon in the upper right and select Settings (Settings).
Step 2: On the interface settings you select Privacy (Privacy) in the left side menu. Under Who can contact me? (Who can contact me?) You click Edit (Edit) on the line I want to filter a message someone on your Inbox? (Whose messages do I want filtered into my Inbox?).
Step 3: Select the option Strict filtering (Strict Filtering).
So you've blocked strangers from adding to the group chat list on facebook in the Inbox section and it will be moved to the Other section .
Good luck!