The SP recent topics/posts blocks are using the SSI equivalent functions to retrieve the data.
These functions comply with the permission rules set for each group, so if anyone doesn't have permissions to see a topic, they will not see it.
So, I guess we could reword your question to: can a block show topics only from certain boards?
If so, then the answer is straight in the page where you set the blocks options: there is a list of all the boards of your forum and you can pick the boards that can appear in a certain block.
That said, yes, there is no option by default to exclude specific boards and keep all the others, mainly because (well, I have no idea why it was originally implemented that way, but I can guess why I would have implemented it that way) an "inclusive" setting gives a more consistent result, in the sense that if you add a new board with an inclusive setting, you are sure the block will not show the board unless you "force" it to, but if you have an "exclusive" setting, then adding a new board will change what the block will show (because the board is not excluded by default).
So: exclusive == more inconsistent, inclusive == more predictable.
If you want it the other way around, it's not that much changes.