What is the solution for entertaining bad thoughts about people and constant obsessive whispering (waswasah)?

Among the things that help you get rid of evil suspicion are: treating people as you would like them to treat you, bearing in mind that the Prophet, peace and blessings be upon him, warned against evil suspicion by saying: "Beware of suspicion, for suspicion is the falsest of speech," and that Allah forbade it in His saying: "O you who have believed, avoid much [negative] suspicion, indeed some suspicion is sin."

As for obsessive whispers (waswas): you must seek Allah's help, make abundant supplications, and strive against your soul to turn away from them. You should also acquire religious knowledge. It may also be beneficial to consult a specialized doctor.