I totally agree. I had never used the block button, but when I decided to for the first time, it had disappeared. I did not want to report the guest and none of the reason options given were applicable.
It's not only a pain in the backside to deal with a guest you don't want to accept but who is determined to book, it can affect your acceptance rate.
I've had a couple of instances where I've had to ask Airbnb to cancel a reservation for me and then the guest has just gone and instant booked again (even though I already told the guest why I was uncomfortable with their reservatoin).
One time this happened and I had to call Airbnb again to cancel it, they told me that it would count as two of my three 'freebie' IB cancellations a year, even though it was the same guest/reservation. The guest then went and booked a THIRD time!
There was another similar situation where the guest rebooked after I cancelled. I then asked him to cancel from his end, which he did reluctantly. Ten minutes later, his girlfriend booked the dates. They thought I wouldn't realise it was the same couple but I did. Some people just don't take no for an answer.
As for IP address, yes this is important. It's SO easy for a guest to create a new/additional account. They do it all the time. I've had several guests contact me with one account and then book with another, telling me that they did it to get a discount for first time users. Airbnb is clearly not very vigilant about preventing this.