There is currently no option to remove the variant if the campaign is active (at least one email has been sent).
If you would like to use only one variant, the workaround is to change the template so that it’s identical to the one you wish to use. In this case, the system would only send the template(s) you need. 

Another option is to duplicate the campaign for the purpose of adding new prospects and keep one variant in the duplicated campaign.

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