One way that I have seen is to use a 2x10 board and nail 16 penny nails thru the board. Bend them so they angle all in one direction. Lean the board from the water to something out of the water (Like a saw horse). The idea is that the turtle climbs up to sun, gets snagged on the nails as it trys to back down, and poof, no more turtle. Make sure they bend over far enough to let the turtle pass over the nails.

I also use a floating turtle trap where they climb up and fall in. A weighted barrel with planks to crawl up to might work.

Good luck.