Hi Oslec Arsua,
is it all activities that are still syncing or just tasks? Check the audit trail for a lead that has a synced.
Is it that there was a sync or is there maybe a campaign with a flow step that is creating a task.
There are 2 different things: the setting in the admin are TASKS that are automatically created, no more than once a day, by Marketo in SFDC for each of the activities that are created in Marketo's log. These tasks are all marked as completed. and they are all assigned to the Marketo sync user. It is strongly recommended not to check these boxes, even the "fills out form". Use interesting moments instead.
But this does not prevent the possibility to use the "Create Tasks" flow steps in smart campaigns. These will also create tasks, but you can choose the tasks status and assignee.