At the risk of alienating myself here, there are a few good designers and a few bad designers and a great many that fall into the middle ground of just average. Before we start telling the OP to put up or shut up shouldn't we see some examples? Make a proper judgement call based on evidence?
If - as it sounds - a template is being used and the designer is randomly changing things just to put their 'stamp' on it, the core design work having already been done in the template, then I'd want to know what the original design looks like, and then compare and contrast.
To the OP I'd say, there are no rules, the design works or it doesn't but your designer should be able to justify their decisions with a coherent and well-reasoned argument and, if necessary, cite studies or research to back it up. After all that, if you still don't like the design, then either ask them to change it the way you want - especially if you are paying the bills - or get a new designer, one whose choices you won't question.