there are sprays but they don't last long. i would recommend getting a roll of spiked deep pile carpet protector and putting it on the floor spike side up. it will be fine for you with shoes on, while discouraging the cats completely. this stuff is easy to clean, while home depots and lowes have this on sale for a few bucks per yard. obviously you'll need to put enough distance down along their path so the cats don't simply jump over it. don't worry, it won't hurt their paws, it's simply uncomfortable for them.

http://www.amazon.com/27X75-PROTECTOR-Multy-Industries-5300057/dp/B000M7S4EK

another idea is to use an air or water spray device that motion or collar key fob activates. put one of them on each side of the doorway standing sentry to keep the cats out, while you the human step over it's detection grid. after a few hits they won't come back for more, so the spray and battery should keep for a while.

http://www.amazon.com/Contech-StayAway-Motion-Activated-Pet-Deterrent/dp/B000DZFFN4

of course, the simplest solution would be to install a door and keep it closed. if you need to keep the door open a crack to hear what's going on upstairs and for air exchange, simply put a long hook and eye door lock on the inside and purposely mount it so keep the door open a few inches while locked. sound & air exchanges, but the cats won't squeeze through. if the gap is ~3" you can still lock/unlock the hook from the eye even from the opposite side of the door by inserting your fingers through the door crack. a door chain or hasp works well, but this hook does a better job of keeping the door ajar if the doorway gets drafty.

http://www.amazon.com/6-Gate-Hook-and-Eye/dp/B00004Z0TU

this is what i use to keep my cats out of the bedroom, while still being able to exchange fresh air and loud conversation between rooms.

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BTW if you keep cats in the same home as a high sanitation work space, you may need to invest in a good mechanical HEPA filter. I have a similar need as you, and got an;

http://austinair.com/products/healthmate-series/

it's expensive upfront, but it's low filter replacement costs over time will make it cheap and low maintenance over the lifetime of your cat. I've had mine over a decade, and it's whisper quiet and works as good as new as long as i replace the drum filter every 5 years, and wash out the nylon pre filter annually (or semi annually depending on the environment) replace the pre filter with a new one every 5 years too. one is included with that replacement filter drum.

http://www.bestfilters.com/Austin-Air-Healthmate-Standard-Replacement-Air-Filter_p_251.html

http://www.bestfilters.com/Austin-Air-Healthmate-Standard-Size-Replacement-Pre-Filter_p_239.html

if you are *coin mining using multi-GPU servers, a HEPA filter may dramatically reduce the amount of fan/heatsink cleaning you will need to do over time.