The recommendations are coming from your selected remote databases. There is not a way to completely turn off the recommendations without de-selecting remote databases. If the unwanted recommendations are appearing on searches that do not originate from Advanced Search, you can remove the remote database from appearing in default search results. That will have the affect of removing recommendations from appearing in search results where a remote database was not specifically selected.
To remove a remote database being searched by default, do the following:
Note: the databases will still be searchable in the Advanced Search