On Mar 18, 5:17 pm, wrote:
We have a house built in the late 60s/early 70s that was clearly
renovated (in about 2004) by an idiot who had no concern for safety or
building codes or anything else.

We removed a heavy glass chandelier with the intention of putting in a
ceiling fan, and found that the box was clearly a plastic one designed
for lightweight lighting fixtures and not anything heavy, and
furthermore, was not secured to anything but instead was just sort of
floating in the celing. In fact, it seemed so flimsy we didn't want to
put the chandelier back up.

So we thought fine, we'll just stick in a lightweight fixture for now
until we can get a pro out to re-do the box, since we eventually want
to replace all the ceiling fixtures anyway.

However, for a few reasons related to how flimsy and badly-installed
the box is, we don't think it's really safe to put anything back on
it--the holes to screw a mounting strip onto are stripped, for one
thing, so they don't really hold securely.

How can we safely cap off the wires (power, neutral, and ground) and
put a cover on the hole in the ceiling until we can get an electrician
out to look at it and fix it? Obviously we don't want to have live
wires there but there's no way we can leave the breaker off
indefinitely.

BTW we have aluminum wiring.

Since the wire is in the ceiling there's nothing to worry about. If
you are worried anyway just make sure the power is off to that wire.
Then use a wire nut or black tape.