It depends on what you mean by an "object" but CTRL works just fine when moving frames and shapes in Scribus 1.4.6 on Ubuntu 17.04.
Just make sure that you press CTRL after you start dragging the frame/shape, otherwise you are invoking the "select alternative" function (that can be used to select things that are under other things).
CTRL doesn't work, however, when you are moving nodes in the Node Editor (at least not in 1.4.6). I think there are a few other areas where it doesn't constrain but I can't think of them just now.