If you've carefully conducted your evaluation and your judgement is that the website in question doesn't have much wrong with it, I think that's a perfectly valid conclusion. Giving out an "A+" grade might be rare in usability evaluations, perhaps, but it's still possible.
Perhaps your concern is that basically saying "Looks good to me" would make for a pretty short and unconvincing report?
If it were my website that were being reviewed, I'd want to know the reasoning behind the evaluation. The ten or twelve usability heuristics, being general principles, are probably pretty vague (and intentionally so), which means different evaluators could easily have different interpretations or perceptions and thus they might find, or not find, different problems in a product. So I'd want to know more specifically what criteria you're using to evaluate the website, and then you can explain how the website stacks up against those criteria. And then even if the website rates well in some area and you can't identify any problems, you might have the opportunity to mention possible alternatives to consider.
For example, you might praise the website's use of accelerator keys to improve efficiency for expert users and you might say that choice of accelerator keys is logical, but you could suggest changing the accelerators to be consistent with, I don't know, say, WordStar (because that's what most of the users are familiar with).