So if you just want to demonstrate the functionality of some kind of security system with your project. You can try following.
Either create a VB.Net Web Application or create a Socket based Client-Server Application.
Where Server/Web Application will allow users to upload text files (.txt) to the server.
Now if a user tries to upload an .exe file, then cancel the connection and show the message of invalid file type. and then add the IP Address of the client to 'Black List' table in database.
And every time before starting upload check the table if the current IP Address is found in the table, if found disconnect and show the message 'You IP Address has been black listed for invalid attempts'.
and then try to upload a .txt file from another computer, and the server will not have that IP Address in the black list table so it will allow the connection.
and try again from the previous computer and it will not allow it.
This way you can demonstrate the basic idea of detecting intruder and blocking further access. and if you do not want this and want to do a real life application. as I said in the Lounge
], you need to define what is Intruder means. after that it is easy to add the IP Address to Firewall.