I don't think the road association would have the jurisdiction to do that.  However you might be able to prohibit bringing personal watercraft in over the road.  

There is a list of ponds where personal watercraft are prohibited in 12 MRS 12071-A, which governs the operation of personal watercraft.  Here's a link:

https://legislature.maine.gov/statutes/12/title12sec13071-A.html

You could try contacting the Penobscot and Hancock Soil and Water Conservation Districts, Inland Fisheries and Wildlife, or any local Lake Association, to see if they have any suggestions.  I did find some information about the pond here:

https://www.maine.gov/ifw/docs/lake-survey-maps/hancock/hopkins_pond.pdf

I don't know if this would help you any:  Look up 12 MRS section 685-C (10) which says

10.  Operating a personal watercraft.  Operating a personal watercraft is prohibited on the following categories of great ponds:  

A. Great ponds located entirely or partly within the jurisdiction of the commission that are identified in an official comprehensive land use plan adopted by the commission pursuant to subsection 1 as being not accessible within 1/4 mile by 2-wheel drive vehicles, with less than one development unit per mile, and at least one outstanding resource value;   [PL 1997, c. 739, §1 (NEW).]

B. Great ponds located entirely or partly within the jurisdiction of the commission that are identified in an official comprehensive land use plan adopted by the commission as being accessible within 1/4 mile by 2-wheel drive vehicles, with less than one development unit per mile, with 2 or more outstanding resource values in fisheries, wildlife, scenic or shore character;   [PL 1997, c. 739, §1 (NEW).]

C. Great ponds and smaller ponds located entirely or partly within the jurisdiction of the commission that are identified in an official comprehensive land use plan adopted by the commission as being not accessible within 1/2 mile by 2-wheel drive vehicles, with no more than one noncommercial remote camp and with a cold water game fishery; and   [PL 1997, c. 739, §1 (NEW).]

D. Great ponds with less than all but more than 2/3 of their surface area in or partly in the jurisdiction of the commission that are identified as being of statewide significance in the "Maine Wildlands Lake Assessment" dated June 1, 1987 prepared by the commission, with 2 or more outstanding resource values in fisheries, wildlife, scenic or shore character and with more than 1/2 of their shoreline in public and private conservation ownership with guaranteed public access for low-impact public recreation.   [PL 1997, c. 739, §1 (NEW).]

Here's a link:  https://legislature.maine.gov/legis/statutes/12/title12sec685-C.html

Hope this helps.

Molly - Good work!  I'm glad you had time to delve into it more than I did.  I do see that Hopkins Pond is one of the Great Ponds listed in that study.  So now the question is whether it meets the "other criteria" to protect it from jet skis.  I'll be interested to learn what your contact finds out when he looks into it.  Keep us posted!

This highlights one of the problems that the Abandoned and Discontinued Roads Commission has seen, which is that there are so many laws (and such complicated ones,) that often law enforcement doesn't know what they can or cannot enforce.  The Maine Statutes used to fit in two volumes - now they take up two shelves.  It's not a bad idea when you call in law enforcement to have a copy of the relevant statutes that you can hand to them.  But then it's a question of whether they will have time to look into how the statute applies.

I have some training and information about fresh water issues I can try to offer and those who have boats may have more information. I believe the intention of preventing the use of water crafts on your pond/lake may need some clarity. If there is milfoil or other invasive plants that are destroying your water, then a watercraft will make it worse. If there is an algae issue or other water quality issues, then that too can make it worse with a watercraft. I recommend you contact https://www.lakestewardsofmaine.org/ as they deal with the DEP and towns about water quality issues.

The other issue is if the watercraft is too close to land and if the boat has a permit. If the boat doesn't have a permit sticker then it's an illegal boat and can be reported to the game warden. In terms of motor boats, being close to land this is another issue. Some motor crafts make waves on the water. Some areas on fresh water have no wake zone. When the owner of the watercraft (with a motor) obtains their annual permit from the town, they get a booklet that discusses the rules about no wake zones.  There is a law about no wake zones to prevent shoreline erosion. In this situation you need to have a NO WAKE ZONE clearly marked. If the water craft goes fast enough to make a wake and the wake hits a shoreline this could be a violation which is reported to the game warden.