If a participant is no longer in your class and you want to remove them from your Remind messages, you can easily do that within your account. Only class owners can remove participants from a class. When a participant is removed, they will be notified they have been removed.
To remove one participant, tap the class you need to remove them from then tap the "People" tab. Find their name in the list then hit the 3 vertical dots next to their name and choose "Remove from class."
If you need to remove all participants in a class, you should archive the class. Just select the class from the menu and tap the name again at the top of the screen. On the class info page tap "Remove from class list" and confirm the change.
Click the People tab on the class and find the participant you wish to delete. When you hover on their name, click the three dots button then "Remove from class."
If you need to remove all participants in a class, you should archive the class and reuse the class code. Click the gear in the upper right for the class you wish to delete. Click reuse class code which will archive all participants and messages in that class and create a new class with the same code, name, and icon. Confirm and you're done!