Yeast can be very persistent beasties. Filtration will remove most but not all of them. Some will always slip through. Cold crashing, racking, filtration will remove a majority of the yeast but there is always the possibility of some being left.
Freezing your must (after cold crash, rack and filtration) might take out any remaining yeast, but there is no guarantee for this.
Problem here is that you appear to be using bread yeast. Quality and ability to ferment are variable for this. From the other post it appears you mead may be around 10% and I've seen bread yeast get to 13% without really trying (with feeding up to 15%). We would require O.G, secondary gravity - before and after racking onto blueberries and current gravity. With this information we might be able to work out of the alcohol % is up over 14% - if so, we might have reached the maximum tolerance for the yeast.
I've just googled UV purifiers and found the following information.
Ultraviolet water purification lamps produce UV-C or "germicidal UV," radiation of much greater intensity than sunlight. Almost all of a UV lamp's output is concentrated in the 254 nanometers (nm) region in order to take full advantage of the germicidal properties of this wavelength. Most ultraviolet purification systems are combined with various forms of filtration, as UV light is only capable of killing microorganisms such as bacteria, viruses, molds, algae, yeast, and oocysts like cryptosporidium and giardia. UV light generally has no impact on chlorine, VOCs, heavy metals, and other chemical contaminants. Nevertheless, it is probably the most cost effective and efficient technology available to homeowners to eliminate a wide range of biological contaminants from their water supply. Recent testing has also shown that UV can be effective at destroying certain VOC's, although we would not specifically recommend the technology for VOC reduction.
UV water treatment offers many advantages over other forms of water treatment for micobiological contaminants. Most importantly, it does not introduce any chemicals to the water, it produces no bi-products, and it does not alter the taste, pH, or other properties of the water. Accordingly, in addition to producing safe drinking water, it is not harmful to your plumbing and septic system.
It does state that it will kill yeast but all information relates to water, not alcohol. Therefore, probably not recommended, but an idea to keep in the back of your mind.