If you don't want your number to be searchable in the Truecaller app, then enter your phone number below including the country code and press 'Unlist.' (i.e. +4690512214). You cannot resubmit your number to the directory.
If you have registered an account with Truecaller, you need to deactivate this first in order to unlist. You can do this within the app under Settings->About Truecaller->Deactivate Account.