Deleting the terms doesn't remove them from the thesaurus database unless you purge them from the database.
To purge the deleted terms from the thesaurus database:
Select the deleted terms you want to purge them.
Click on the Purge button in the Edit Properties group of the Term Properties ribbon tab page (or click on the Purge All button if you want to purge all of the deleted terms.)
A warning box will be appeared; click on the 'Yes' button to complete the purge operation.
Notice: This operation completely removes the term and the information cannot be restored.