plt=Graphics3D[{Sphere[{0, 0, 0}, 1], Sphere[{2, 3, 5}, 1]}];
Append[plt, Boxed -> False]
Edit1:
If we have useused:
plt=Graphics3D[{Sphere[{0, 0, 0}, 1], Sphere[{2, 3, 5}, 1]}, Boxed->True]
the above codes fails. To make them more general, we can use:
Append[DeleteCases[plt, Boxed -> _], Boxed -> False]