How to drop all requests using mod_security

The mod_security action deny will produce a 403 response whereas drop will immediately close the connection. From the manual:

drop

Description: Immediately initiate a "connection close" action to tear down the TCP connection by sending a FIN packet.

Action Group: Disruptive

Example: The following example initiates an IP collection for tracking Basic Authentication attempts. If the client goes over the threshold of more than 25 attempts in 2 minutes, it will DROP subsequent connections.

SecAction initcol:ip=%{REMOTE_ADDR},nolog
SecRule ARGS:login "!^$" \
    nolog,phase:1,setvar:ip.auth_attempt=+1,deprecatevar:ip.auth_attempt=20/120
SecRule IP:AUTH_ATTEMPT "@gt 25" \
    log,drop,phase:1,msg:'Possible Brute Force Attack"

Note

This action is extremely useful when responding to both Brute Force and Denial of Service attacks in that, in both cases, you want to minimize both the network bandwidth and the data returned to the client. This action causes error message to appear in the log "(9)Bad file descriptor: core_output_filter: writing data to the network"

Apache has to handle the request in some way, it can't just ignore them. Responding with 403 Forbidden is as close as you get.