OK, this is where it gets very very silly.
Firstly let me stress that there a lot of really good reasons to secure your network, not least of which are privacy of your machines and data, viruses spreading, and even costs (you may pay for usage of your internet!). If you have fire-walling facilities allowing you to run a DMZ (de-militarized zone) for public wifi that is safer.
However, the Digital Economy Act has just turned things on their head slightly. It actually encourages you to run an open wifi.
The logic goes like this...
1. Running a public access wifi hotspot is 100% legal - the act does not make it in any way wrong to do that. You may not think so reading it as you can lose an appeal against technical measures if you cannot show you took steps to secure your wifi, but it does not actually require you to take such steps, and if you read on - failing to do so could make you immune from technical measures anyway...
2. Once you are running a public access wifi you are clearly providing a communications service (to the public even). This makes you a communications provider by any reasonable definition of that phrase (and by definitions in the Communications Act).
3. Tell your internet provider that you are now buying the service from them as a communications provider. Well, best to ask first in case they have any extra terms, AUP, or costs, but some (like AAISP) don't mind at all. In fact go further - advise then that sending you copyright infringement reports would cause you distress and alarm.
4. Now you have done that you are not a subscriber as defined by the Digital Economy Act 2010. But as you have no agreement with the public for them to use the wifi so you are not a service provider either. So you don't have nasty obligations under the Act either.
5. Your internet provider cannot treat you as a subscriber as the definition is clear in the Act. As such they have no legal requirement to pass on or count copyright infringement notices to you for your IP addresses. In fact, if they do they could be comitting an offence under Protection from Harassment Act 1997, section 22 of the The Privacy and Electronic Communications (EC Directive) Regulations 2003, or section 127 of the Communications Act 2003.
6. The internet provider must not count the notices they get or take technical measures against you under the Digital Economy Act because you are not a subscriber.
In fact this means you are safe from all of the measures of the Act, basically.
Now, the copyright holders could get your details by use of an expensive court order and take legal (civil) action against you - but you now have the perfect excuse - you run a public wifi so in all probability the copyright infringement was carried out by some random member of the public. After all, you are already on record that you run a public access wifi. You have created plausible deniability.
Obviously this is not legal advice, but sounds pretty simple to me - the new Digital Ecomony Act actually encourages people to run public wifi to make themselves immune to the copyright infringement reports, technical measures, and even direct civil cases.
Maybe it is a cool and useful act after all? Well done labour.
P.S. AAISP have made the situation clear for its customers http://aaisp.net.uk/legal-cp.html
P.P.S Why not make the public wifi only work on IPv6 with RADV. That way you are not allocating IP addresses either :-)