It's not likely to fully block the effects of Methamphetamine or other stimulants, but I think it would be important for your own safety to know why you're taking the Levetiracetam (Keppra) for. I can only guess that your taking it for seizure prophylaxis, so this would really make using powerful stimulants like Methamphetamine contraindicated. Not only is acute usage probably a bad idea, but if you lose control and find yourself in a compulsive binge, you risk seriously messing with your seizure threshold.
Some more information would be swell, but it would appear that there's not a direct, serious interaction in either a positive or negative way.