Whether it's a keyword, a URL, or an entire campaign, you can always delete things, and add in new ones. Whatever limit your account has regarding how many keywords you can use, it's just a limit at one specific time.
If you're limited to 100 keywords at a time and you've filled those up, you can delete 10 of those and enter 10 new ones whenever you want. Similarly, if you have a client entered in for one of your campaigns and you lose that client, you can easily delete their entire campaign and fill it with something else.
Some people have asked if they can delete all of their sites every week to make room for new ones, to spread their link building across a massive amount of sites over the course of a month. While you could technically do this, it's really not recommended because of the way we order link building. It would be very tough to keep track of exactly which sites have received how much link building, since Link Emperor is designed to be a longer-term strategy where you receive well-rounded link building over time by rotating through different services on the same sites. If you keep swapping out sites, your link building will be extremely haphazard for those particular sites and it probably won't be enough to see any kind of ranking improvement, either.