Note: Long post ahead.
You're right on a couple of points, but the problem with spam won't go away. They'll just find another forum to spam, and thus something else will be taken away. The cycle will repeat until finally the entire forum is reduced to a place where even posting a welcome message will get you deleted, and then they win.
There are two tpes of spammers, newbies who don't know better, and souless spammers who just want to make trouble. The first tpye eventually learn to not spam, but the second type will never go away. They'll continue spamming until they win.
I'm speaking from experience when I say that taking away the things many people love to do JUST because spammers have decided to take control will, at most, just slightly slow down the problem. They will just keep comming back and attacking until they either get tired or run out of IPs.
On top of that, when you take away something like the sprite shops, where many people had fun and enjoyed the "feature" (best way I can describe it), you also run the risk if making some people mad and turning them away from your forum, which would be a bad thing as well. As you take away the enjoyment, you turn people away. Slowly but surely, the community will die out as people leave to join a forum that has the feature(s) that they enjoy.
You can't win by taking away good things from good people because the losers that are determined to fuel their own evil ego want to run loose. As you give in to their attacks, they slowly win.
My suggestion: Give them less of a reason to be evil. Eventually they'll either get bored of attacking and leave on their own or else they'll run out of ways to spam.
Also, about Axel and Roy's problem with load times, that's what sig rules are for. Edit them as needed to have people use fewer sprites and images.