are there devices to prevent theft of electric distribution lines or cables?

Answer 1

Yep... we call them chain-link fences with razor wire, but a group of professional soldiers could breach one in about three seconds and disrupt our power grid so bad and so fast that the whole country could be plunged into darkness for days by less than 500 men, maybe way less. Let's just hope the terrorists don't figure that out.

Answer 2

There are some alternatives :
A. ACTIVE SYSTEM:
1. By using current limiter device.
In this case you have to know how much power do you have to provide for your legal user(s). When more power draws from your power source exceeding the capacity of the limiter, the device will trip. Then you may investigate the cause(s).
2. From time to time, you must do electrical load audit (when the time is suitable), by swithcing off all eligible loads. Please observe whether or not there is illegal load(s) by checking the current meter installed in your source.
B. PASSIVE SYSTEM:
1. By enclosing the cable into metal conduit (expensive).
2. By using thick PVC pipes as conduit (cheaper, but breakable).

Answer 3

Yeah, put electricity through them...that usually stops people just picking them up....