If you don't want to see results from a specific site on DuckDuckGo, you can block that domain from appearing in your search results. To do this, click the ••• icon next to a result from the site you want to block, and select "Block this site from all results". This will block all links to this domain from appearing in your search results, as well as Instant Answers that source content from that domain. You can add up to five domains to your block list, which can be managed at any time from Search Settings.
If you ever want to temporarily see blocked sites in your search results, without removing them from your block list, you can do so by selecting "Show all results" at the top of the search results page. This will add any blocked sites back to where they would normally appear in your results. Select "Hide blocked sites from results" to remove these sites from your results again.
If you just want to exclude a domain from a single search, instead of blocking it permanently, you can just add -site:[domain] to your search query, for example -site:wikipedia.org. This will not add the site to your block list, and only exclude it from appearing in your current search results. Conversely, you can use site:[domain] to exclusively see results from a specific domain.