Is a mouthguard effective at treating teeth grinding?

Possibly but... A mouthpiece would really be the choice. A "mouth guard" is often too bulky and may create more muscle spasms and grinding. A mouthpiece alone can be mildly effective, but you need some muscle therapy and some exercises to try and maintain your issues.
No. It will protect your teeth from damage if you do clench or grind due to the plastic and because it helps to balance the stresses of doing so over the entire chewing system. However, the appliance itself will not "stop" you from grinding your teeth. These is known to be induced from causes other than the teeth.
Teeth Grinding. In general the right types of mouth guards are highly effective. Typically store bought guards are too flexible and actually cause more clenching. In addition, more people actually clench their teeth than grind them.