@gamerouche, it's probably your browser that is remembering the search terms you tapped in earlier. Not the forum software.
The way to get rid of the search history differs from one browser to another. Generally, go to Settings, History, delete search history/fields/something else. But note that in some browsers you may also lose your logins, since those are also fields you filled in. Make sure you have a record of login names and passwords somewhere safe (like on paper). Need to know which browser you're using to give you concrete practical advice.