You can both cease to crawl a seed or seeds on a regularly scheduled basis or stop a current crawl from running at any time.
If you no longer want a specific seed to be crawled, you can deactivate by going to the clicking on its hyperlinked URL in its collection's "Seeds" tab, then uncheck the check-box next to its status as an "Active Seed." Press the "Save" button at the bottom of this interface and the seed will no longer be crawled according to any regular schedule. To apply this change to multiple seeds at once, click the check-boxes next to each in the "Seeds" tab, then on the "Edit Seeds" button above them. In the dialog box that opens, toggle the radio button for the seeds' status from "Active" to "Inactive" and click the "Apply" button to make your change, after which none of them will be crawled on any regularly scheduled basis.
To halt an actively running crawl from continuing, access the report for that crawl from the "Current Crawls" tab in the "Crawls" section of the web application, then click on the "Stop Crawl" button under that crawl's "Overview" tab. For more information on monitoring these actively running crawls, see: How to monitor your crawls.