Is there any simple way of blocking undesirable internet usage on a home wifi network?
In a nutshell.
NO. Anyone who tells you to the contrary doesn't know what they are talking about.
Anything simple is going to be simple to bypass.
You're going to have to put some degree of effort into anything worth doing.
If you want something simple... get a second broadband line with BT Retail (assuming you are not currently a BT Retail customer)... and allow BT to enable the Openzone public hotspot feature.... that way, you will force people to have their own BT Openzone account and are therefore legally accountable for any actions taken under their own Openzone account.