Соответствующая норма
Belgium
Practice Relating to Rule 161. International Cooperation in Criminal Proceedings
Section C. Extradition of own nationals
In 1973, during a debate in the Third Committee of the UN General Assembly, Belgium noted that Belgian law prohibited the extradition of Belgian nationals. 
Belgium, Statement before the Third Committee of the UN General Assembly, UN Doc. A/C.3/SR.2022, 9 November 1973, § 40.