No chair should have arms, they're horrible for you. Fortunately it's pretty easy to take them off the Aeron. First remove the back of the chair. There are 4 large hex bolts that hold the back on to the two posts coming up from the seat. Those come off easily. Now you can see the inside of the sliding arm assembly. Each arm is held on by one bolt. These are star-head ("torx") bolts which nobody has the tool for. Fortunately if you jam a hex key in there really hard and press firmly while you turn you can get these off. The arm tightening wheels will spin so you have to hold them in place to unscrew these bolts. The sliding arm assembly is sprung and the tightening wheels have ball bearings, so you want to be careful as you get close to taking these bolts out - try to hold it all together as you pull the bolts so it doesn't all go flying. The arms should fall right off now and you can put the back back on.
LOL on the negative side I keep reaching for the arm to brace myself when I sit down and almost just wiped out when I reached and grabbed air.
Also a small bonus, the really ugly Aeron looks much better without arms.