Everyone on Instagram can message you, whether they follow you or not. However, messages from people other than your followers are kept under a separate folder (Requests) in DM.
While Instagram doesn’t let you stop DMs for normal messages, you can restrict DM for stories. Instagram offers three settings for message replies in stories: Everyone, People you follow, and Off.
Here’s how to set it.
Step 1: Open Instagram Settings by tapping the three-dot icon (Android) and gear icon (iPhone) on the profile screen.

Step 2: Tap on Story controls and under Allow message replies, select the preferred option.

