You can take two approaches: canonical or noindex/nofollow.
The advantage of rel=”canonical” is that any link equity from external links to the variation URLs will be transferred to the original URL. The disadvantage is that Bing/Yahoo may not recognize the canonical tag.
Noindex/nofollow, applied either as a page tag or in robots.txt, is universally recognized, but will negate any link equity from external links to the variation pages.
You can decide which of the two approaches would work best for you. In either case, these can serve as temporary measures, since you can (and should) apply hard 301 redirects to your content experiment pages once a test concludes and you have moved the winning variation to the original URL.