How to stop Spam from existing users with few posts?

[HLFadmin]

How many users are listed in ACP / Users and Groups / Groups / Administrators / Options / List Users
This should show all the groups the listed members belong to.
Are you the only one listed?

Then look at ACP / Users and Groups / Admin Permissions
You can revoke permissions of any user who is not superadmin.
Are you the only one listed?

What does ACP / Tools and Maintenance / File Verification show?
List the modified files here.