What is the ruling on renting out internet access to a group of households while there is doubt about their using it for sinful acts, and what is the ruling on monitoring their devices without their knowledge?

Benefiting and helping people are good deeds for which a Muslim is rewarded, such as "delivering a useful service to your brothers."

However, service is a double-edged sword, so users must be warned against using it for disobedience and threatened with its termination if that is proven.

There is no blame upon you for allowing them to use it as long as nothing contrary to the Sharia appears from them, and it is not permissible for you to spy on them.

If it becomes clear after the contract that they are using it for unlawful purposes, and you did not stipulate this to them, then the contract remains valid, and you do not have the right to cut it off from them.