They caused financial damage – make them pay for it. If they are minors, make their parents pay and publish their names. If those paintings are really damaged and they are really valuable, the damage is going to be very expensive to cover. Make sure the activists remain on the hook for the entire sums plus the cost of police and the judicial system to process them. Or, just take everything away from them and make them work for it. They might not care about the money but an aching back plus hurting feet have a very strong educational effect. By the looks of it, they have not worked a day in their lives.
Vandalising, risking damage to irreplaceable artworks, because none of us have listened to their spoiled tantrums.