iRule to allow only POST method

Try this;

when RULE_INIT {
    set static::maxRate 10
    set static::windowSecs 3
    set static::timeout 30
}
when HTTP_REQUEST {
    set method [HTTP::method]
    log local0. "http request is of $method"
    if { $method eq "POST" } {
        set postCount [table key -count -subtable [IP::client_addr]]
        log local0. "postCount=$postCount"
        if { $postCount < $static::maxRate } {
            incr postCount 1
            table set -subtable [IP::client_addr] [expr {int (rand() * 10000)}] "ignore" $static::timeout $static::windowSecs
            return
        } else {
            log local0. "This user $user has exceeded the number of requests allowed."
            HTTP::respond 501 noserver Content "Request blockedExceeded requests/sec limit." Retry-After [expr {int (rand() * 100)}]
            return
        } 
    } else {
         Return "Method not allowed"
        HTTP::respond 405 noserver
        return
    }
}