Your organisation will need to have agreed policies and specific rules which your unpaid volunteers, and any paid staff, are required to comply with.
Your organisation's agreed policies might include:
use of email and internet
bullying and harassment
alcohol and drugs
You will also need to have set rules and policies on:
disciplinary procedures (incuding your organisation's agreed convention on what constitutes gross misconduct)
Written copies of your organisation's policies and procedures should be given to employees with their contract as part of a staff handbook and covered as part of their induction training.