New Musescore - Hide meter, etc.,

Uncheck the Visible button in the Inspector

There are different ways to hide these things depending on your specific reason for wanting to hide them and how you want the rest of the score to be formatted.

Simply clicking an element and pressing "V" (or using the Inspector to uncheck the "Visible" property) works, but it leaves a "hole" where the element used to be, not what you want most of the time, unless your goal was to maybe to write in those elements by hand later after printing.

If you simply don't ant any key or time signatures anywhere in your score, you can turn them off by right clicking a staff, Staff Properties, and unchecking the appropriate options. Some are in "Advanced Style Properties".

If you are trying to supress a courtesy signature at the end of a line specifically but for a reason *other* than starting a new movement, click the actual signature on the next line and turn off the "Show courtesy" property in the Inspector.

If you are starting a new movement, simply use a Section Break from the breaks palette at the end of the previous movement, and courtesy clefs and signatures are automatically suppressed, plus the measure numebring is reset, etc.