How can a girl set boundaries for herself in her interactions and stop joking and smiling inappropriately with young men, especially since she regrets doing so?

Joking and laughing with non-mahram men is not permissible for a woman, as it is no less than the forbidden "softening of speech" (khudu' bil-qawl) that is prohibited by religious law. As Allah Almighty says: "Then do not be soft in speech, lest he in whose heart is disease should covet." Some scholars have explicitly stated that a non-mahram woman laughing with a non-mahram man is unlawful and necessitates disciplinary action (ta'zeer). Therefore, one must repent to Allah for this act, bearing in mind Allah's observation and His wrath, and striving for modesty, protection, chastity, and shyness. It is also advisable to associate with righteous women and avoid mixing with men except in cases of extreme necessity, while adhering to the jurisprudential controls in speech, and emulating the Mothers of the Believers and the righteous women of the Ummah, and increasing in supplication.