After taking a look over BoonDocks TJMod ( http://wolfwiki.anime.net/index.php/User:Eft-BoonDock#Change_amount_of_ammunition ) I would bet there is a way to do this. At least you could “steal” the ammo from any other weapon than the one you want to force the player to use.
-- all classes
WP_LUGER = 2 -- Luger
WP_COLT = 7 -- Colt
WP_AKIMBO_LUGER = 38 -- Akimbo Luger
WP_AKIMBO_COLT = 37 -- Akimbo Colt
WP_MP40 = 3 -- MP40
WP_THOMPSON = 8 -- Thompson
WP_GRENADE_LAUNCHER = 4 -- Axis Grenades
WP_GRENADE_PINEAPPLE = 9 -- Allies Grenade
-- soldier
WP_PANZERFAUST = 5 -- Panzerfaust
WP_FLAMETHROWER = 6 -- Flamethrower
WP_MORTAR = 35 -- Mortar
WP_MOBILE_MG42 = 31 -- Mobile MG42
-- medic
WP_MEDIC_SYRINGE = 11 -- Syringe / Adrenaline
WP_MEDKIT = 19 -- Medi Pack
-- engineer
WP_KAR98 = 23 -- Axis Engi Rifle
WP_GPG40 = 39 -- Axis Rifle Nade
WP_CARBINE = 24 -- Allies Engi Rifle
WP_M7 = 40 -- Allies Rifle Nade
WP_PLIERS = 21 -- Pliers
WP_DYNAMITE = 15 -- Dynamite
WP_LANDMINE = 26 -- Landmine
-- fieldops
WP_AMMO = 12 -- Ammo Pack
WP_SMOKE_MARKER = 22 -- Airstrike
-- covertops
WP_STEN = 10 -- Sten
WP_FG42 = 33 -- FG42
WP_K43 = 32 -- Axis Covert Rifle
WP_GARAND = 25 -- Allies Covert Rifle
WP_SATCHEL = 27 -- Satchel Charge
WP_SMOKE_BOMB = 30 -- Smoke
et.gentity_set(clientNum, "ps.ammoclip", 10, 0)
et.gentity_set(clientNum, "ps.ammo", 10, 0)
The last 2 lines changing the ammo of sten (entity ID = 10, see above) to zero.