Can a trustee refuse to give disbursements to a beneficiary under a living person's trust?

No. The trustee only has the power granted in the document that created the trust. If the trust was drafted properly, there are provisions in the trust document for the removal of the trustee and for appointing a new trustee. If not, then the matter can be brought before a judge and the court can remove a trustee who is violating the terms of the trust.