They make special screws called Security screws. They require a special screwdriver bit to unscrew them. Here's a Link,

Link

I would suggest that you use these kinds of screws to secure them. While it's true that any thief could order there own set of security drivers, it would require an extra step over the spur of the moment kind of theft.


If you really want to make your installation at one time thing, use a grinder to grind off the heads on regular screws. Just gring off enough so there nothing left to get a screwdrver on it, but not enough to take off the head of the screw. Doing this will require anyone removing the panels to use a sawsaw or cutting torch to get the screws out. Also the damage to the panels screw holes would make them less portable for resale. Try to get flush screw heads as possible and coat everything with grease when your done, so they can't get a pliers on the screw head.    

Another suggestion would be to have trees like Douglas-Fir's in front of the solar panels blocking the direct line of sight from the road. 

You could also get a security camera pointed at the panels and have to record to computer. You could get a package setup for a few hundred dollars. 

In the end nothing is going to pervent a determined thief for stealing anything they want. The Best you can do is try to make things more difficult for them.