Sirium.net does use a firewall for (perhaps obviously) security reasons. This means that it can and will block you if you repeatedly try logging in to certain services, and fail, within a certain period of time.
So if you fail to log in to cPanel 5 or more times within 10 minutes, your IP address will be added to the firewall's block list.
This may be a pain for yourself, but it is purely to try and keep the server (and your data) clean and safe, and your web site online.
There are a number of things that can cause you to be blocked by the firewall. The most common one is repeatedly trying to log in to a service with incorrect details, as mentioned above.