Yes, blowing or dumping leaves and any other yard waste into the lake is illegal. It is considered fill and requires a permit under RSA 482-A for the NH Department of Environmental Services. if observed please takes photographs of the activity and submit them with a copy of a completed complaint form which can be found at:
https://onlineforms.nh.gov/app/#/for...5-5204b86175e7
The address to send them to is listed on the form.