I've had a KIN with a $10/75MB data plan for about a month and a half now. You don't want to block data, because you're already paying for 75 MB that you won't be able to use if you do that. Here's how it actually works (or at least how it has consistently worked for me):
If WiFi is enabled and you are connected to an available WiFi network, then the phone will default to WiFi and will not use the Verizon 3G network. I know you may have read things to the contrary, but that's the way it has worked for me EVERY TIME. The only time my phone has used Verizon's network was when WiFi was not available. So far, in the six weeks or so I've had the phone, I've accumulated a little over 8MB in total Verizon data usage -- and none of that came as any surprise.
The tricky part is that you need to frequently check to make sure you're actually connected to WiFi.
-- The world (and your house) are full of things that emit radio signals that can interfere with typical WiFi signals. Things like cordless phones, various remotes, even microwave ovens all emit signals that can reduce the effective range of your WiFi router. (There is one room in my house where whenever the cordless phone rings, it knocks my daughter's laptop offline.) So any time I'm using data for more than just a quick hit, I check the dashboard frequently to make sure I'm still connected to WiFi. If you've lost the WiFi connection, then the phone will try to use 3G.
-- When I leave home, I usually turn off WiFi to conserve battery power. If you do the same thing, then when you turn it back on somewhere else, look closely at the connection setting (not just the dashboard) when you check it. I find that when I first turn WiFi back on, it will say it's connected -- but still to the WiFi at my previous location! It takes a few seconds for the phone to realize that the previous network is no longer available. Then it will rescan and connect to a local WiFi if one is available.
-- When you get to a new place, the phone will scan to find available networks. It will list them, but if this is a place you have not connected before, you still have to tap the network you want in order to connect to it.
-- When you check the connection, make sure that it doesn't just list the name of your network, but specifically says "Connected to <name of network>". The type is pretty small, so make sure it isn't stuck on "Connecting to...", or doesn't just list the network without saying anything.
The first time I tried to connect in a public place, the phone connected to the network in a local mall, but when I tried to check my e-mail, I kept getting the dreaded "Cannot connect to server" message. I checked the settings, and the connection seemed fine. I finally gave up, shut off WiFi and let the phone use 3G to check my mail. The "problem" turned out to be very simple. Many unsecured public networks still require you to go through a screen where you accept their term/conditions before it will pass you through to the external internet. I hadn't done that yet. But as soon as I brought up the browser and tried to go to an uncached page, I got the mall's login screen. Once I got past that, everything worked fine. The interesting thing about this is that as far as the phone was concerned, it was connected to WiFi. Because of that, it would NOT use 3G until I turned WiFi off. So once again, my consistent experience has been that the device will default to WiFi if it is connected, and only use 3G when it's not. No offense to anyone, but I've got to believe that the couple of folks who racked up hugh data connection charges just weren't paying attention and assumed they were connected to WiFi when they weren't.