When new orders arrive, they are assigned to a server in a round robin manner. If one of the servers is getting full and you'd like to disable any new accounts from being placed on that server, you can:
Navigate to Setup -> Products -> Servers
Click the Server to stop future signups on.
Enter 1 for the "Maximum # of Accounts" field.
This will fill signify that the individual server is full, and it will not allow any more signups.