I have never worked with AceSEF, so I have no idea about its capabilities features and such.
I guess what you want is to not add those urls of the snippets items in the sitemap, since they are not actually pages.
If the sitemap creation feature does not provide a way to exclude items/urls from the sitemap, then one possible way to achieve this would be to override this feature.
Unfortunately I have no access to the component, therefore it is impossible to speak specifically about what you have to change on the component.
But you could dig into the extension's sitemap creation code and find out more about how it is being handled.
A possible approach could be the following:
You could create a specific category called "Snippets", and then you could exclude all its items from appearing in the view part of the sitemap. It is possible that this exclusion must happen at a helper file, which might query the database for items, or inside a jSeblod - AceSEF plugin.
This is how I would started, in conjunction with a question to the AceSEF developers to find out more about the AceSEF's approach.