I cannot fathom the idea of buying anything for the sole purpose of keeping other from having it. Why? Why be so utterly vindictive towards people you will never meet? Why tie money up in the sole pursuit of being an ass?
Now, having said that- doing something that benefits you at the expense of someone else? I can understand that to an extent. Buying things to flip, for example- you're denying someone else a chance at a deal, but at least there's a reason... you want to turn a profit. It's not an attempt to actively hurt another collector, it's just you trying to help yourself out. Now, trying to buy all of a limited thing to control the supply & slowly drip them out for ever-increasing profits is a dick move- but it's also very risky. You may have the current supply, but in such a situation I imagine the people who own the rights would likely re-release the item, killing your resale value.
Personally, I enjoy when you can split the difference on things. For example- years ago, I found 2 Sega CD memory carts at a flea market. One was in box for $5, one was loose for $1. I knew someone online I was going to see at a con soon who wanted one, so I sold her the loose cart for $25 (I think). A tidy profit for me, but about half of the Ebay price at the time for her- everybody wins!