How do you guys handle rejections after telling your potential clients that you need their password? 
Every time a client ask me the question for the security im calm and telling them that they dont need to worry, after saying this the most of the time its good
What do you guys do?
Find out what they are afraid of and reassure them. You could also they them that you guys have a common goal and everything you do on the account will benefit them. Could you give more context?
You most likely just need to spend more time talking with them to build trust. You will need that trust to keep them as a client for a long period of time anyway.