Securing a WiFi connection

We are considering installing an internet connection in our two church halls. Initially this is intended for controlling heating system as the hall is hired out and it would be more convenient for those who have to set up the heating times to do so remotely.
However, some groups hiring the hall could want internet access for business reasons.
How can we ensure at reasonable cost that it is not misused - i.e. used to access porn or other inappropriate sites?
The simple obvious way is to give the group a guest Id and Pw which is cancelled after the hire but that doesn't control what they do during the time they have access. Also it would not be practical for regular users of the hall.

The risk is very small given the groups that use the hall, but the vicar is somewhat paranoid and I need a reasonably convincing reassurance argument!

Unless you have anything on the router its hard without using a lot of money the cheap and effective option is to run a proxy server but its how practical that is but it does let you restrict sites and also logs all users activite which provides evidence Install Free Proxy Server on Windows - YouTube

The Vicar has every right to be concerned, he will be held accountable for access to adult/inappropriate sites. Access to Web sites would be difficult to control, even to trusted users. As far as heating and cooling control, programmable thermostats are relatively cheap and don't require user intervention once they are programmed.

I would look at Circle:
Meet Circle: Parental Controls & Internet Filtering