When you don't want a particular translator to translate any of your future requests, you can block that translator by adding them to your Blocklist.
When a translator repeatedly did not meet your expectations with his/her translations, and you want to add the translator to your Blocklist, go to the feedback page of your finished translation requests, find the translator, and select 'Block' in the drop down list of 'Add to list'.
By doing this, the translator will not be able to see your future requests anymore.
Please note that when you block a translator, you prevent him/her from seeing any requests you make for a language pair. However, if you have Japanese to English requests and English to Japanese requests and you don't want any of them to be seen by the translator, you need to block the translator for each language pair.
You can unblock a translator from the left sidebar on your screen. Click Translators, then the Blocklist button on the Translator lists page. There will be a list showing all the translators you have blocked.
Click on the 'x' button right next to the translator's language pair. By doing this, the translator will be unblocked.