Dave D. I can imagine the rage you must have felt about finding the dead turtles w/ holes in their shells. I feel for you.
The shitty thing about that specific incident is that the guilty party was just killing for fun.
As I'm an absentee land owner myself, I have had the most success in busting people at my place from Mondays through Thursdays, from early afternoon to dusk. This is not as much fun though as you can't be doing your regular pond activities, but instead have to be hidden somewhere.
Another first step (as Meadowlark has said) is to make the bastards aware that you are aware of their activities. This alone does stop some individuals.
While these items have most likely been mentioned before, here's a list of things I would do:
1)Spread the word to certain local folk that you are having trespassing problems.
2) Gather up the turtle shells and keep them in a place where they can be seen.
3) Start expending hundreds of rounds of ammo from a handgun or rifle every time you go to your property for the next month or so. Leave some empty shell casings around.
4) Hang targets w/ bullet holes at various locations around your property. I use old coffee cans hung from trees.
5) Leave a note for these bitches. I like to let them know that when I do catch them, I will be pursuing the most severe action/penalty within my rights and under the law.
6) If possible, donate some amount of money to the local police and let them know what you're going through.
7) If you haven't already, make efforts to find some local person who for a few bucks can act as a caretaker type. Obviously it's hard to know who you can trust at this stage. This is also very helpful in narrowing down the windows of time when the trespassing is happening.
Again, I'm sure you know all of this.
