The 25-year-old tested positive for Methylhexaneamine following the League Two side's match against Dagenham & Redbridge in December.
He was provisionally suspended by the club in January.
Marshall, who signed for the Bees in 2010, will not be able to return to action until 2014.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Barnet | 12 | 9 | 0 | 3 | 25 | 11 | 14 | 27 |
| 2 | York City | 12 | 7 | 4 | 1 | 18 | 6 | 12 | 25 |
| 3 | Forest Green RoversForest Green | 12 | 7 | 3 | 2 | 20 | 9 | 11 | 24 |
| 4 | Rochdale | 12 | 7 | 2 | 3 | 20 | 9 | 11 | 23 |
| 5 | Gateshead | 11 | 7 | 2 | 2 | 22 | 15 | 7 | 23 |
| 6 | Oldham AthleticOldham | 12 | 6 | 5 | 1 | 17 | 11 | 6 | 23 |
| 7 | Eastleigh | 12 | 6 | 3 | 3 | 19 | 15 | 4 | 21 |
| 8 | Sutton UnitedSutton | 12 | 5 | 4 | 3 | 19 | 16 | 3 | 19 |
| 9 | AltrinchamAltrincham | 12 | 5 | 3 | 4 | 15 | 15 | 0 | 18 |
| 10 | Dagenham & RedbridgeDag & Red | 12 | 4 | 5 | 3 | 20 | 13 | 7 | 17 |
| 11 | Halifax TownHalifax | 12 | 4 | 5 | 3 | 12 | 10 | 2 | 17 |
| 12 | Yeovil TownYeovil | 12 | 4 | 3 | 5 | 13 | 14 | -1 | 15 |
| 13 | Woking | 12 | 4 | 3 | 5 | 13 | 16 | -3 | 15 |
| 14 | Solihull MoorsSolihull | 12 | 4 | 3 | 5 | 14 | 18 | -4 | 15 |
| 15 | Aldershot TownAldershot | 12 | 3 | 5 | 4 | 17 | 17 | 0 | 14 |
| 16 | Southend UnitedSouthend | 12 | 3 | 5 | 4 | 17 | 17 | 0 | 14 |
| 17 | Tamworth | 11 | 4 | 2 | 5 | 12 | 19 | -7 | 14 |
| 18 | Hartlepool UnitedHartlepool | 12 | 3 | 4 | 5 | 7 | 11 | -4 | 13 |
| 19 | Maidenhead UnitedMaidenhead | 12 | 3 | 2 | 7 | 14 | 20 | -6 | 11 |
| 20 | Braintree TownBraintree | 12 | 2 | 4 | 6 | 6 | 13 | -7 | 10 |
| 21 | Boston UnitedBoston | 12 | 2 | 3 | 7 | 12 | 22 | -10 | 9 |
| 22 | Fylde | 12 | 2 | 3 | 7 | 13 | 24 | -11 | 9 |
| 23 | WealdstoneWealdstone | 12 | 1 | 5 | 6 | 9 | 17 | -8 | 8 |
| 24 | Ebbsfleet | 12 | 1 | 2 | 9 | 9 | 25 | -16 | 5 |
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