If you run WordPress on Windows (and who doesn’t?) and have the need to block specific user agents (bots, crawlers, browsers) below is a decent way I’ve found that uses rewriting rules and works along side the needed WordPress rules:

Adding this rule to your web.config will block the request from the specified agent(s):

 <rule name="RequestBlockingRule1" stopProcessing="true">
          <match url=".*" />
          <conditions>
            <add input="{HTTP_USER_AGENT}" pattern="agent1|agent2|agent3" />
          </conditions>
          <action type="CustomResponse" statusCode="403"
             statusReason="Forbidden: Access is denied."
             statusDescription="You do not have permission to view this page." />
        </rule>

So your entire web.config may look like this:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <system.webServer>
    <rewrite>
      <rules>

        <rule name="RequestBlockingRule1" stopProcessing="true">
          <match url=".*" />
          <conditions>
            <add input="{HTTP_USER_AGENT}" pattern="feedburner|android" />
          </conditions>
          <action type="CustomResponse" statusCode="403"
             statusReason="Forbidden: Access is denied."
             statusDescription="You do not have permission to view this page." />
        </rule>

        <rule name="WordPress Rule 1" stopProcessing="true">
          <match url="^index\.php$" ignoreCase="false" />
          <action type="None" />
        </rule>
        <rule name="WordPress Rule 2" stopProcessing="true">
          <match url="^([_0-9a-zA-Z-]+/)?wp-admin$" ignoreCase="false" />
          <action type="Redirect" url="{R:1}wp-admin/" redirectType="Permanent" />
        </rule>
        <rule name="WordPress Rule 3" stopProcessing="true">
          <match url="^" ignoreCase="false" />
          <conditions logicalGrouping="MatchAny">
            <add input="{REQUEST_FILENAME}" matchType="IsFile" ignoreCase="false" />
            <add input="{REQUEST_FILENAME}" matchType="IsDirectory" ignoreCase="false" />
          </conditions>
          <action type="None" />
        </rule>
        <rule name="WordPress Rule 4" stopProcessing="true">
          <match url="^" ignoreCase="false" />
          <conditions logicalGrouping="MatchAny">
            <add input="{REQUEST_FILENAME}" matchType="IsFile" ignoreCase="false" />
            <add input="{REQUEST_FILENAME}" matchType="IsDirectory" ignoreCase="false" />
            <add input="{URL}" pattern="([a-zA-Z0-9\./_-]+)\.axd" />
          </conditions>
          <action type="None" />
        </rule>
        <rule name="WordPress Rule 5" stopProcessing="true">
          <match url="^[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*)" ignoreCase="false" />
          <action type="Rewrite" url="{R:1}" />
        </rule>
        <rule name="WordPress Rule 6" stopProcessing="true">
          <match url="." ignoreCase="false" />
          <action type="Rewrite" url="index.php" />
        </rule>
      </rules>
    </rewrite>
  </system.webServer>
</configuration>