What is the ruling on maintaining computer devices in internet cafés that are used to access forbidden websites, knowing that their owners are able to block these websites but do not do so?

It is permissible to repair computers in internet cafes unless it is known that their use is restricted to the forbidden. If the use is unknown, or if they are used for both the permissible and the forbidden, then it is more prudent to avoid repairing them. It is forbidden to repair them if the predominant use of them by people is for the forbidden, because the ruling is based on the predominant use.