Podle mě by to mělo být takto:
  <system.webServer>
    <httpErrors>
      <remove statusCode="404" subStatusCode="-1" />
      <error statusCode="404" prefixLanguageFilePath="" path="/" responseMode="ExecuteURL" />
      <remove statusCode="500" subStatusCode="-1" />
      <error statusCode="500" prefixLanguageFilePath="" path="
http://semtopresmerovat.cz/" responseMode="ExecuteURL" />
      <remove statusCode="403" subStatusCode="-1" />
      <error statusCode="403" prefixLanguageFilePath="" path="
http://semtopresmerovat.cz/" responseMode="ExecuteURL" />
    </httpErrors>
    <security>
        <ipSecurity>
            <add ipAddress="46.23.0.0" subnetMask="255.255.0.0" allowed="false" />
            <add ipAddress="93.91.0.0" subnetMask="255.255.0.0" allowed="false" />
        </ipSecurity>
     </security>
  </system.webServer>