I am not a lawyer, but I think that the interpretation of -SA varies wildly. In this case, there is a clause in the text of -SA which applies here:

When You Distribute or Publicly Perform the Work, You may not impose any effective technological measures on the Work that restrict the ability of a recipient of the Work from You to exercise the rights granted to that recipient under the terms of the License.

Therefore, making it technologically (or legally) challenging for end users to use your adapted versions is a violation of -SA (to my understanding).

Nice one!

@Boom I actually did do a little work on a green goblin baddie a while ago, but was never quite satisfied with its club, and got caught up with other stuff. Here's a draft of the idle animation. I also made a version without the ears which gives it more of a green cave-troll kind of look.

Definitely not, you would have no legal standing, and yes that would be very selfish. Artists here work hard to produce content that they are choosing to share with the community. The entire purpose would be lost if someone could come and literally snatch that work out from the community. Consider the original artist - would THEY still be able to use their own work, after you have copyrighted it (which again, you cannot do)?

If you want artwork that nobody else can copy from your original game, then you must create it yourself or negotiate a deal with an artist to create it for you. Note there are several highly talented artists here who advertise their services for fees, and a whole commissions forum devoted to connecting artists with developers http://opengameart.org/forums/open-commissions