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>