Hi! Are you able to click into one of the profiles added, and see how they were imported? A common culprit could be an integration syncing in profiles from another platform.

If you have a campaign you wanted to send out immediately, until you can get this list removed, you can get a lower Klaviyo plan that allows sending to only your list of 400 people. Then you can create a list or segment for just the people you have uploaded, and send to that only.

For a more long-term solution, you could make a segment of everyone added after a certain date (a date after your last legitimate import). Make sure you exclude anyone who signed up from a legitimate source, like if you have a Klaviyo signup form running somewhere. 

Once you have a segment of addresses you are 100% sure you would like to remove, convert to a list (in the dropdown in the upper right corner under Manage Segment). Download the list as a CSV (so you have a backup, JUST in case you find you do want any of these addresses). Then, go to Settings → Other → Profile Maintenance, and select the list of profiles you want to remove. 

Again, I am always VERY careful when removing profiles. You don’t want to accidentally delete the people who are in your account legitimately! 

Hope this helps, and let me know if everything made sense or if you have any questions!