In my design, there are some lines with "=" on them. Can someone tell me why these lines appear and how to delete them? I'm using Altium18.
Those lines are error markers that are there to show that the net is not connected with a trace in copper (open circuit).
From the Tools Menu, select "Reset Error Markers".
That should solve your issue
This is one of the more annoying misfeatures of altium. Error markers for un-routed nets that are redundent with the rats nest lines and don't dissapear as you route the nets or move around as you move the components, they just stick where they are until you run the next design rule check.
As Elmesito says you can do a "reset error markers" but that will remove all violation markers.
You can also run a design rule check with "un-routed nets" unticked but then you won't get them in the report either. I'm not sure if there is a way to get it to include un-routed nets in the report but not to add those stupid redundent violation markers for them.
Quick trick: if you want to check for un connected net, just press ALT and select over your whole board. This selects connections (unconnected)