If a girl doesn't like a proposal, can she reject it? I have always wanted to get marry to a person who has more knowledge and practices Islam more than I do. If I found the proposal from a person with modern values, can I reject this proposal? I prefer Istekhara for this, but still I want to know whether it is permissible or not in Islam.
Absolutely, she may reject a proposal, and there's numerous sahih hadith which state this, e.g.:
Narrated Abu Haraira: Allah's Messenger (ﷺ) said, "A lady slave should not be given in marriage until she is consulted, and a virgin should not be given in marriage until her permission is granted." The people said, "How will she express her permission?" The Prophet (ﷺ) said, "By keeping silent (when asked her consent)." Some people said, "If a man, by playing a trick, presents two false witnesses before the judge to testify that he has married a matron with her consent and the judge confirms his marriage, and the husband is sure that he has never married her (before), then such a marriage will be considered as a legal one and he may live with her as husband." -- Sahih al-Bukhari 6970 (sunnah.com)
Narrated `Aisha: I asked the Prophet, "O Allah's Messenger (ﷺ)! Should the women be asked for their consent to their marriage?" He said, "Yes." I said, "A virgin, if asked, feels shy and keeps quiet." He said, "Her silence means her consent." -- Sahih al-Bukhari 6946 (sunnah.com)
The Prophet (ﷺ) said, "A matron should not be given in marriage except after consulting her; and a virgin should not be given in marriage except after her permission." The people asked, "O Allah's Messenger (ﷺ)! How can we know her permission?" He said, "Her silence (indicates her permission). -- Sahih al-Bukhari 5136 (sunnah.com); also Sahih al-Bukhari 6968 (sunnah.com)
One of the functions of her wali (guardian) is to only marry her to a suitable husband, so he can also reject a proposal on her behalf.
Marriage without having the consent of the guardian of a woman is not valid. -- IslamWeb
It is haraam for the guardian (wali) of the woman to force her to marry someone she does not want and does not like, because the Prophet (blessings and peace of Allah be upon him) said: “The virgin should not be given in marriage until her permission has been sought.” Narrated by al-Bukhaari, 6968; Muslim, 1419. -- Islam Q&A
And if a marriage does happen without her permission, she can seek annulment:
With regard to what has happened to you, you have the choice of whether to stay or not. Seek guidance from Allaah (by praying istikhaarah). If you agree to stay in this marriage then all well and good, but if you do not accept to stay with your husband, then you have the right to seek annulment of the marriage, because it took place without your consent. -- Islam Q&A