You can prevent future connections:
REVOKE CONNECT ON DATABASE thedb FROM public;
(and possibly other users/roles; see \l+ in psql)
You can then terminate all connections to this db except your own:
SELECT pid, pg_terminate_backend(pid)
FROM pg_stat_activity
WHERE datname = current_database() AND pid <> pg_backend_pid();
On older versions pid was called procpid so you’ll have to deal with that.
Since you’ve revoked CONNECT rights, whatever was trying to auto-connect should no longer be able to do so.
You’ll now be able to drop the DB.
This won’t work if you’re using superuser connections for normal operations, but if you’re doing that you need to fix that problem first.
After you’re done dropping the database, if you create the database again, you can execute below command to restore the access
GRANT CONNECT ON DATABASE thedb TO public;