Please try the following steps.
1. Add an explicit ACL for traffic going to the that particular server & enable blacklist on the ACL (as shown in screenshot).
2. Please enable blacklisting under the Security profile on the SSID
SSID-->Security--Blacklist
Now in case any user is trying to send traffic to that server , it should get
dynamically blacklisted.
The user mac address can be checked from the monitoring page (Alert) as seen in the screenshot.
The blacklist time can be changed as well as seen in screenshot
System--Show advanced--Blacklisting
We also have an option to log the acl & hits can be seen under security logs.
As indicated from the post, the client is not allowed to access that URL, so its better to blacklist it .