This might come as a surprise for you, but the project manager may chair the change control board (CCB) if there is no potential conflict of interest.
If the CCB is chaired by the PM, then he would have no veto power. However, if for example, the CCB is chaired by the Chairman or the CEO, then most likely the CCB chairman will possess veto power. In either case, the procedure by which the CCB approves or rejects changes should be well documented in the change management plan (which is part of the project management plan).
Typically the CCB consists of a mix of technical and business people. Technical people will assess the technical impact of the change on the project and will weigh in with their opinions, while business people will assess its financial impact on both the project and the organization. The CCB does not necessarily have to include top executives, just people with enough expertise and authority to make decisions on the project.