Maybe you could modify one of the Hasp Type hardware setups to secure the cap.
Generally, someone looking to vandalize will use the things they find onsite, unless they are targeting you and bring something with them. So making as few things as possible available in the immediate area of where the tractor is stored might be a good idea as well.
Also, depending upon the barns location, you could also use either security cameras or even game cameras to keep a good eye on things. At least if you had a suspicion there was activity in the barn that shouldn't have been you could check the camera before starting the tractor to be safe.
While pulling the tank and flushing it is a hassle, it's far better than running an engine with fuel that's purposely been contaminated.
My brother, who is in charge of the county Jail and a Captain with the Sheriff's Dept, borrowed one of my Harley's and rode it to OFFICE DEPOT. When he came out and rode home, the bike started acting funny and running rough. We went back to Office Depot and had them play back their security cameras. Turns out one of the guys my brother had put in jail saw him ride up on the bike and dumped a fountain drink into the tank after he went inside. The tank had to be replaced and the engine rebuilt. State Farm covered the cost, which totaled almost $5,500. With the security tape as evidence, the guy was arrested, charged with Felony Vandalism and being he was a three time loser, is now in prison for a long, long time.