Hi, Pilou,
I don't see a way currently to hide the lines, there's seems to be a .hidden property for them, but it doesn't affect the visible display of the lines.
For the delete commands, the script has to be rerun or maybe setup like this in the Options dialogue to delete with no gui :

K ConstructionLine

delete last line:
Alt+K script:var gd = moi.geometryDatabase; var clines = gd.getObjects().getConstructionLines(); if ( clines.length > 0 ) gd.removeObject( clines.item(clines.length-1) );

delete all lines:
Ctrl+K script:var gd = moi.geometryDatabase; gd.removeObjects(gd.getObjects().getConstructionLines() );

Hope that helps --

Paul

PS --- I see Michael beat me to it. Disregard mine :)