iRule for disallow Request Content-Length in the header from being sent

It's better if the dev team fix their code to send the correct content length value.

If that's not possible, you can disable the violation from your ASM blocking settings.

Removing the header via irule might trigger other violations.

But you can also recalculate the length in an iRule:

when HTTP_REQUEST {
    if { [HTTP::method] eq "POST" } {
        HTTP::collect
    }
}
when HTTP_REQUEST_DATA {
    HTTP::header replace Content-Length [HTTP::payload length]
}