Usually you will not need this as the program tracks where you have successfully submitted and will uncheck the sites for you when you load a project. If you wish to submit to certain sites, use the Selection Tool. Click Main Menu → Tools or Tool bar (left of Provider List).
For example, select only German providers with a page rank better than 0 using these steps:
If you load a new project, the selection is of course gone again, so you can load/save the selection for a later use.