I lock a spare hitch ball in the receiver on the TT and lock it with a "shank less lock" which has no place for bolt cutters to get a grip. The key slot is in the end. It ain't foolproof, but it would take a lot of work and noise to remove.
2013 Silverado 2500 HD Duramax/Allison Tuned
Aviator Wright Flyer #1919
"First In, Last Out". 11B4Y
DAV, MOPH, VFW, 5/7 Cav,1/8 Cav Assoc.
11th ABN Pathfinder Co. N7MIP