Need to stop sub site users from changing their theme and disable wordpress dashboard!

Hi

I am currently getting the full dashboard appear to my sub site owners (please see screen shot). How can I edit this so that message is dismissed before they even see it? In its place it would be nice if I could add my own little introduction explaining what they need to do next to edit their store and add their products. Is that possible?

They are also being allowed to edit their theme! This option really needs to be disabled to these users. Can anyone help?

I’ve been hoping I could use the ‘user role editor’ plugin, but nothing seems to work with that. For instance I have manage_categories unchecked, but users still seem to be able to add their own categories to the store, which I want to stop. Why can they do this if it’s unchecked?

Could any one help me with these points please?

Thanks

Sally