captive
adjective [ before noun ]
COMMERCE uk/ˈkæptɪv/ usnot having a choice about what services, goods, etc. you buy because there is only one or there are only a few available in the place where you are:
In the cinema, advertisers know that they have a captive audience.
Long-distance airline passengers are the perfect captive market.
Utility companies could be fined for overcharging captive customers or undercharging consumers who have a choice of supplier.