I'm slightly aghast at the suggestion that slinging a low chain across a trail to stop motorcycle riders might have been better, especially after the mention of someone being decapitated in the same posting. Whilst a low chain might not decapitate someone, it could still cause a serious accident resulting in death or serious injury.
Whilst personally I dislike motorcycle riders tearing down trails in designated wilderness areas, setting wires, chains or other barriers is repugnent. This type of behaviour should be condemmed outright and those responsible brought to justice.
Those that ride motorcycles are out to enjoy themselves and whilst they should find somewhere more suitable to engage in their activity, they do not deserve to be maimed because someone feels they have the moral high ground. If you engage in chain slinging etc, you have lost the moral high ground outright.
The previous posters suggestion of taking pictures, contacting authorities etc is a much more appropriate course of action. Bike seizures, heavy fines, imprisonment for extreme cases, education campaigns, land set aside for motor cyclists etc are much better solutions and tactics to take.
Roger, whilst i'm glad to see you do not support extreme action yourself, your last comment 'A lower chain might have been better' is not helpful. How can anything slung across a trail that can cause injury be 'Better' ?
This sort of vigilante action needs condeming. Full Stop.