Hackers constantly pull my login.aspx

Question

I have a website with protected content and I've recently started getting a lot of email alerts about unsuccessful logins. It gets really annoying, about one attempt in a minute or two on average.

Because I host the website on a dedicated Windows box and I log the IPs, I realized that these requests are not coming from regular visitors to my site (because statcounter doesn't record the IPs), but from automated scripts from many different IP addresses (mostly from Ukraine, but mainly Reverse DNS cannot locate them).

I created a blocking rule in Windows Firewall and started to add all the addresses I found in the log file, but they are many. I already added probably 50 (5 times by 10 IPs) and this is stopping them only for few hours and then new IPs start coming around.

I am actually a software programmer and managing a real server is not my best side. Are there any tools that I can use to prevent these attacks?