I've been playing with Simple Share Buttons and the Pinterest plugin together and have found 2 solutions!
Solution one, unfortunately I cannot release until I have finished my current development. All share buttons images will have the class attribute - 'ssba', so you will be able to use the 'exclude classes' feature offered by the pinterest plugin.
For now, there is another solution that you could go for - the Selected images only feature - you'd need to add the class 'pinthis' to all those images that you wish to make 'pinnable'.
Really though, these share button images shouldn't be picked up by the Pinterest plugin at all. The minimum size of a pinnable image is 81px squared - http://pinterest.com/pin/56224695318345544/ so ideally these images should not be picked up by the plugin at all.
I'd say it's worth raising with the plugin author in their support forum as it should be addressed really.
Hope this helps!