To mass delete activities first start by going to the activity list at the top of your screen.
Next you can use the filtering options to find what you want to delete.
Once you have the list of activities you want to delete you will then go to the top right corner of the screen and use the tagging option to "tag all filtered".
This will then check/tag everything you had filtered out for deletion. Once you have everything tagged you can then use the delete tagged option to mass delete them.
This will then prompt you to confirm you want to delete them and you will hit confirm.
Once you click confirm the system will then mass delete these out of the system.