If you are running Windows firewall:
Start>Control Panel>Windows Firewall.
On the Exceptions tab, click 'Add Port...' and enter the details:
Name: blockland
Port: 28000
TCP/UDP: TCP
Do it again, but this time click UDP, see below:
Name: blockland
Port: 28000
TCP/UDP: UDP
While you are there, check to see if Blockland.exe is on the list. If it isnt, click 'Add Program...' and search for Blockland.exe and add it.
It seemed to not matter whether windows firewall was on or off, but this fixed it.
I am not sure what to do for vista, but shouldnt be too different.
As for the banned thing... There is a guy going round on servers, and he has a way to ban people from their own servers. What happened on my server:
[Guy1] Watch out for [Guy2] cause he knows how to ban ppl from their own servers.
[Guy1] He just came on our clan server and banned the host!
[Guy1 has left the game]
....
[Guy2 has joined the game]
[Guy2] I know how to ban people from their own server.
[Guy2] This is what you do: [cut out because I don't want it to spread]
[Guy2 has been banned forever ""]
Not exact spelling, but yeah.
The method is strange, I have no idea how he came across it.
Names have been blocked out.
Guy1 is a well respected member of the community.
Maybe this is what happened to you?
