How can one deal with extremists who make people doubt Islamic banks, even though there is acceptance and a positive impact in calling for them?

Continue inviting people to goodness, enjoining what is right, and forbidding what is wrong. Do not stop because some people question Islamic banks. Explain what you know to those who ask you about the transactions of the bank you work for. Patience in enduring people's harm is better, and avoid disputes and enmity. Assume the best of those who criticize Islamic banks, as some of them may not adhere to Sharia regulations. Whoever knows something should clarify it without making incorrect generalizations. Encourage Islamic banks and call for their reform and for advising those in charge of them.