The air rifle is a good idea, but only if you also have a rabbit fence: while they are distracted trying to dig under, they might make easy targets (assuming you have time to lie quietly in wait late in the eve)!!!
Proper answer - rabbit mesh is the only means! I am by no means an expert in this area, but if they are desperate to chew your greens then a 900mm high mesh with 150mm pegged out-turn is the recommended standard. Forestry Commission reports that digging the 150mm into the ground is no more effective than a 150 out-turn: a turn-out obviously involves a lot less work.