You want to measure the FG of a beer that's already been carbonated, or you are worried about the de-gassing of CO2 in primary/secondary when you take hydrometer readings?
If you want to measure the FG of a beer that's already carbonated, just degas it, like suggested above. If you are worried about the slight bit of residual CO2 in primary/secondary when you take your FG reading, just make sure you spin your hydrometer so it doesn't have any CO2 bubbles on it.
Even easier: Do you have an OG measurement for this brew? You could get pretty close on estimating the FG and then ABV by just using the expected attenuation rate of the yeast you used without having to measure FG. For example an S-05 yeast (and most ale yeasts) has a 70-75% attenuation rate, so your FG is going to be 70-75% of your OG. You can then use that estimate to estimate the ABV with a margin of error of about +/- 0.2-0.3 or so.
Don't overthink it. I can't think of many good reasons why a home brewer would need an EXACT ABV measurement, and you can estimate it pretty easily.
Also, long run, spend $22 and get BeerSmith. Among the 50+ other brewing calculations it automates for you, it does a OG estimate, FG estimate, attenuation estimate, and ABV estimate based on your ingredients. You can then manually enter OG and/or FG actual readings to automatically fine tune the ABV measurement, among countless others.