"Stop screwing around." This phrase means the exact same thing as "Stop messing around" or "Stop playing around." It means act more serious, or focus more on what is important and not other things that don't matter.
For example, a boss might tell his employees "Stop screwing around!" if they are all being lazy and not working.
"Screw around" can basically mean anything non-important or non-focused. For example, if someone asked me what I did today I might answer "Oh I just screwed around."
"Screw around with..." can mean two different things. The first is a negative meaning. If I say "Don't screw around with me," I am saying I want you to stop the stupid, annoying, or rude thing that you are doing.
BUT there is another neutral meaning. If am using something (my camera for example) in an unfocused way I might just say "I'm screwing around with my camera."
Hope that helps.