Brazilian Citizens:

If you are a Brazilian citizen you must enter Brazil on a Brazilian passport.

Brazilian citizens need not/cannot apply for visas to Brazil.
Brazilian citizens do not need the permission of the Brazilian government to enter Brazil.

Dual Citizens:

If you are a dual-citizen of another country and of Brazil, you must enter Brazil on a Brazilian passport.

Brazilian citizens, even though they may also be citizen of other countries, nee not/cannot apply for visas to Brazil.
Brazilian citizens do not need the permission of the Brazilian government to enter Brazil.

Example:

Q - I have Japanese citizenship and Brazilian citizenship. Do I need a visa to go to Brazil?

A - No, you may enter Brazil using your Brazilian passport.