You can clear notifications one at a time or in groups by selecting the square next to the notifications you want to interact with. When you do you will have the option at the bottom of the screen to mark them read or delete them.

Clearing a notification about an activity does not affect an activity.

Step 1: Click the check box next to a notification or multuple notiications.

Step 2: Press Delete to remove them, or you can choose to Mark them Read.