You may be held accountable for their activities while using your broadband - you would be the one getting the warning letters for infringements if current legislation goes though.
Certainly a wireless router that can do "client isolation" is a good idea as otherwise they can infect each other being on a shared LAN (malware can spread much more easily this way than from the internet alone, especially if they are not all up to date with their patching).
Some wireless routers can create separate SSIDs each with a different password which would also be better than tenants sharing one. I'm guessing DD-WRT and similar custom firmwares can do this.
Otherwise, advising them to turn off file sharing on the wireless interface of their laptops would help, but does not eliminate entirely the means for malware to spread.
NAT means that internal laptops would not be visible directly on the internet, so internet malware necessarily has to rely more on exploited websites / infected downloads / attachments / social engineering rather than a direct attack.
There is an argument, despite the initial cost overhead, for putting the tenants on a second line with its own broadband so if they use up all their allowance it does not use up yours. This could be on a monthly rolling contract if needed. It would be neater from a legal/tax standpoint to have your domestic broadband separate from the broadband being provided as a landlord/lady.
Newnet for example do let you set a fixed or flexible cap so you can have more control of the total cost, or maybe you could go for something like Be on a suitable package that won't lead to unexpected costs.
Have you added any wording to the contract that the tenants have some (shared) responsibility not to abuse the free/ inclusive service, or it might be withdrawn for one or all at any time?
Just a few thoughts, not wanting to knock any of the ideas expressed earlier.
prompt $P - Invalid drive specification - Abort, Retry, Fail? $G
prlzx on n e w n e t Max ADSL