Praise be to Allah.
Al-Bukhaari narrated in his Saheeh from ‘Aa’ishah (may Allaah be pleased with her) that the Prophet (peace and blessings of Allaah be upon him) would never leave anything in his house on which there were any crosses without erasing them or – according to another report –cutting them out.
This hadeeth indicates that if the Prophet (peace and blessings of Allaah be upon him) saw a cross on something, he would not leave it without blotting it out, or cutting it out if it was still visible after blotting it out, because the cross is something that is worshipped instead of Allaah, and its presence is something objectionable (munkar) which must be changed. This is a summary of what was said by Al-Haafiz in Al-Fath.
It was also reported that the Prophet (peace and blessings of Allaah be upon him) said:
“Whoever among you sees an evil action (munkar), let him change it with his hand [by taking action], and if he cannot, then with his tongue [by speaking out], and if he cannot, then with his heart [by feeling that it is wrong], and that is the weakest of faith.”(Narrated by Muslim).
Muslim also narrated that Abu’l-Hayaaj al-Asadi said: “ ‘Ali (may Allaah be pleased with him) said to me: ‘Shall I not send you on the same mission that the Messenger of Allaah (peace and blessings of Allaah be upon him) sent me? Do not leave any image without blotting it out and do not leave any built-up grave without levelling it.’”
These two ahaadeeth indicate that it is obligatory to change, remove or blot out any objectionable thing (munkar). The symbols of kufr (including the six-pointed star) are included in this, so they must be removed and wiped out. And Allaah knows best.