You need the support of someone in the organization that has the authority to establish a global policy or that has the ability to win support throughout the organiation for such a policy. In short, an executive sponsor or champion. This might be the CIO, CTO, VP or Director of IT. There also needs to be an effective plan in place for dealing with exceptions and criteria for approving and rejecting requests for additional space. Provided that such a policy is adopted, you will need the tools to enforce the policy. Last but certainly not least by any measure, the users will need to undergo awareness training, as well as receive instruction on making efficient use of storage so that they can comply with the policy.