I have my contact form set up to throw an exception when a spam attempt is made so now instead of receiving a spam, I receive an exception report.

Code XML:
Exception information:
    Exception type: System.Web.HttpRequestValidationException
    Exception message: A potentially dangerous Request.Form value was detected from the client (message="com1, <a href="http://prof...").
Request information:
    Request path: /Contact
    User host address:
    Is authenticated: False
    Authentication Type: 
    Thread account name: NT AUTHORITY\NETWORK SERVICE
It's always the same two IPs:
User host address:
User host address:

I'm on shared hosting and don't have access to IIS control panel. Can I block these IPs without affecting performance for non-spammers?