Except you would have to dedicate as much time to it as it would lock someone out. For instance, 5 bad attempts take you 0s + 1s + 2s + 4 + 8s = 15s and locks them out for the next 16s. So, if you wanted to lock someone out for a day, you have to spend a day (less one second) locking them out. Even if you automated this attack, surely IT could handle that.
IRV is a step in the right direction, but there is math which shows that it tends towards the same two party system we have now. There is also empirical evidence to back this up. I am for voting reform, but I think IRV would essentially be shooting ourselves in the foot. If it were successful, we wouldn't see the kind of change we want (i.e. better representation) and the two party system would be strengthened. After all, we would have IRV and they would still be winning (to the chagrin of many of their "constituients"...)
Outside of a dog, a book is man's best friend. Inside of a dog, it is too dark to read.