Open the wbs file(the map) with notepad and set playable civ =1 like here and maybe minorciv to zero:
(Could be you have to do the opposite for another civ)
BeginPlayer
Team=6
LeaderType=LEADER_WILLIAM
LeaderName=Justinian
CivDesc=Byzantine Empire
CivShortDesc=Byzantine Empire
CivAdjective=Byzantine
FlagDecal=Art/Interface/TeamColor/byzantine.dds
WhiteFlag=1
CivType=CIVILIZATION_DUTCH
Color=PLAYERCOLOR_ORANGE
ArtStyle=ARTSTYLE_EUROPEAN
PlayableCiv=1
MinorNationStatus=0
StartingGold=800
StateReligion=RELIGION_ISLAM
StartingEra=ERA_MEDIEVAL
CivicOption=CIVICOPTION_GOVERNMENT, Civic=CIVIC_FEUDAL_MONARCHY
CivicOption=CIVICOPTION_ECONOMY, Civic=CIVIC_FEUDAL_AGRICULTURE
CivicOption=CIVICOPTION_VALUE, Civic=CIVIC_SOCIAL_ORDER
CivicOption=CIVICOPTION_RIGHTS, Civic=CIVIC_ARISTOCRACY
CivicOption=CIVICOPTION_RELIGION, Civic=CIVIC_STATE_PROMOTION
AttitudePlayer=2, AttitudeExtra=7
AttitudePlayer=10, AttitudeExtra=9
AttitudePlayer=11, AttitudeExtra=-47
AttitudePlayer=13, AttitudeExtra=-98
AttitudePlayer=16, AttitudeExtra=-43
Handicap=HANDICAP_NOBLE
EndPlayer