filtering traffic with irule

You can use an Local Traffic policy to do this:

https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/local-traffic-policies-getting-started-12-1-0/1.html

Or a simple iRule:

when HTTP_REQUEST {
    if {!([HTTP::header exists "my-header-name"])} {
        reject
    }
}

Even if you request a Drop, I would suggest to send a 403 response page instead!

Take the lee’s Answer and replace reject with

HTTP::respond 403 content {put here the html content of your blocking page}