Enable Edit Mode first (see Edit Mode).

A single symbol, line segment, or custom object can be hidden by using the context menu and use Hide.

Now the symbol is gone.

If you use the CTRL key and keep it pressed you can also create an area to select multiple symbols. The symbols will be marked with the boundary box during this process.

Then you use Hide again.

And now they are also gone.

At the end of the process disable the Edit Mode again (see Edit Mode).
 

With the Hide / Show toggle button...

... you can show the hidden symbols again. The hidden symbols will be shown with a boundary box so you can distinguish them from the symbols which are not hidden.

Enable Edit Mode first (see Edit Mode).

To show symbols again you use the toggle to show the hidden symbols and then use Show.

If you also select some symbols which are not hidden, it isn't a problem.

And now they are all back.

At the end of the process disable the Edit Mode again (see Edit Mode).