Get a surveyor and shoot that line. Post it every 50 feet no trespassing. Put a fence up just do something.

1- Get the line set.
2-Post it.
3-Fence it.
4-Take pictures.
5-Set cameras up.
6-HOA sends registered letters to the trespassers.
7-HOA files a trespassing complaint with the police.

These items will cost very little to complete. The trespassers will have no say as they can not show ownership. The HOA has the upper hand at this point. Laying down will just give them an advantage. You cant waste time with these kind of people. Taking action on your own rights will make them think. What are they going to do? Call the cops. Also have the legal description and ownership proof available in case they do try calling the cops. Right now you have the right to keep them off. The police as a rule will not want to be involved. However they should be able to be on the side of the legal tax paying owners.