To the first question, you can open a terminal and type:
Now the reminder dessapears from Planck.
To the second question, you can try to show all the hidden startup applications. Open a terminal and type:
sudo sed -i 's/NoDisplay=true/NoDisplay=false/g' /etc/xdg/autostart/*.desktop
Then launch the Gnome session properties from terminal:
Look for the appcenter launcher and uncheck it. It called:
Source of some parts of this answer by @cipricus:
How to access and edit Startup applications in Elementary OS Freya beta 2?