Web-Server shows multiple event id 4625 from one IP

On a web-server running our web-site there are multiple Login attempts like the one listed below from IP address. I blocked the IP address with a firewall and all was good until I received word that one of our users was unable login to our web-site. After talking to this person it turns out they are a legit person that should be able to access our web-site. The problem occurs when they log into the password protected pages on our site. I worked with this persons IT staff and found that the user had the check-box "Register this connections address in DNS" checked. We unchecked the box and for a week all was good. Then we started to see the same problem. After talking with the IT person again we tried un-checking the checkbox "Append parent suffixes of primary DNS suffix"(a shot in the dark) and that worked for a couple of days but again the problem is back now. This is the only person with this problem out of 10,000+ valid users. Also it attempts this login through several random ports.

I am new to networks and network security and not able to determine what the problem is and how I would go about auto-blocking the IP addresses of machines that try to log in to the web-server like this with 50 to 100 attempts. Also a way for the user to fix the problem on their end.

Any help would be greatly appreciated

-Grajek

###############Event LOG Below##################

An account failed to log on.

    Subject:
    Security ID:        NULL SID
    Account Name:        -
    Account Domain:        -
    Logon ID:        0x0

Logon Type:            3

Account For Which Logon Failed:
    Security ID:        NULL SID
    Account Name:        xxx-xxxx$
    Account Domain:        XXXXXXX (this value is the users domain and not our domain)

Failure Information:
    Failure Reason:        Unknown user name or bad password.
    Status:            0xC000006D
    Sub Status:        0xC0000064

Process Information:
    Caller Process ID:    0x0
    Caller Process Name:    -

Network Information:
    Workstation Name:    XXXXX-XXXXX
    Source Network Address:    XX.XXX.XXX.XXX
    Source Port:        63312

Detailed Authentication Information:
    Logon Process:        NtLmSsp
    Authentication Package:    NTLM
    Transited Services:    -
    Package Name (NTLM only):    -
    Key Length:        0

This event is generated when a logon request fails. It is generated on the computer where access was attempted.

The Subject fields indicate the account on the local system which requested the logon. This is most commonly a service such as the Server service, or a local process such as Winlogon.exe or Services.exe.

The Logon Type field indicates the kind of logon that was requested. The most common types are 2 (interactive) and 3 (network).

The Process Information fields indicate which account and process on the system requested the logon.

The Network Information fields indicate where a remote logon request originated. Workstation name is not always available and may be left blank in some cases.

The authentication information fields provide detailed information about this specific logon request.
    - Transited services indicate which intermediate services have participated in this logon request.
    - Package name indicates which sub-protocol was used among the NTLM protocols.
    - Key length indicates the length of the generated session key. This will be 0 if no session key was requested.