If you are in the ISP or hosting business it is vital to have an AUP. Without one you leave yourself open to legal challenges from problem customers. Spammers specifically seek out hosts with badly-written or no AUPs because it means they can abuse your network and threaten you when you try to get rid of them.
You can read our own AUP at following link: http://domainregister.international/knowledgebase/47/AUP---Acceptable-User-Policy.html
You can create your own AUP in a simple way, using Spamhaus's free tool: https://www.spamhaus.org/isp/aup_builder/