Maybe I'll convince some Jihad scum that the machines are the new "big thing", and we can take care of two birds with one stone.
Phased plasma rifle in the 40 watt range
Unless they are running on some sort of AI matrix, disruption of communications would be a good start, since they would be likely remotely operated. Perhaps a faraday cage - copper mesh?
Wait for its battery to run down and then shoot whoever comes to recharge it. Battery development is lagging quite a bit behind the robots. If they go solar power the panels would be easy enough to damage with small arms fire. Hydrogen fuel cells could be a problem for us if they are sufficiently developed in the near future, but they have to run out of fuel some time even then.
I think fire would also be effective. Would at least damage sensors or seriously mess up any heat vision/infrared.
They probably wouldn't be so uparmored to maintain mobility. Could probably damage them with good old steel core 7.62x54 or any sort of AP round. .50 BMG would give them a whallop as well.
Ask it to calculate Pi to the last decimal.