Depending on your hosting environment there are a few options via the web.config file.

One way that I use for similar reasons is the rewrite rules element of the system.webserver section.  Replace the pound signs with the IP Numbers you wish to block.  Be careful and make sure to back up your config file just in case things go off the rails.  You can add multiple remote addr entries using this approach and regular expression matches to close out whole IP ranges.  You can block out huge areas very easily so you will want to make sure the regex expressions are right.  

<system.webServer>

<rewrite>

<rules>

<rule name="Block access from unwanted REMOTE_ADDR">

<match url=".*" />

<conditions logicalGrouping="MatchAny">

<add input="{REMOTE_ADDR}" pattern="\b###\.###\.##\.###\b" />

<add input="{REMOTE_ADDR}" pattern="\b58\.49\.(25[0-5]|2[0-4][0-9]|1?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|1?[0-9][0-9]?)\b" />

</conditions>

<action type="AbortRequest" />

</rule>

</rules>

</rewrite>

</system.webServer>