This could get a bit messy, but I think the best way to do it is to import the people to a static list. You could always create a dummy acquisition program ("DEL-Client Emails" for example) that you use when importing the contacts to the static list. This will make it easy to delete them after you're done with the email send.
Once you set up the email in the program (I suggest having a unique identifier for those programs so that you don't wrap them up within your reporting), send the email either via the email program, or via a smart campaign. If they aren't looking for reporting and you want to keep everything organized in one program, you can set up the emails and the smart campaigns within DEL-Client Emails so that you don't have a bunch of extra programs floating around.
Once the email is sent and you don't need the contacts anymore, you can run a smart campaign that deletes everybody with the acquisition program "DEL-Client Emails". If the people are supposed to remain in your system because they were already existing, they won't have "DEL-Client Emails" as their acquisition program so they won't be deleted.