Assumption we talk about symbol pins here.
ERC is just a tool to check your work. It can only do so much. If you set all pins to bidirectional then you reduce its power.
Not in the symbol. Place the not connected on purpose flag (blue cross in the right toolbar) within the schematic.
Every pin declared as input expects to be connected to an output. This includes power input pins. And as power symbols are implemented via power input pins also these. For power input pins read the FAQ article already posted by @paulvdh.
The short version is that there is a special symbol with a single power output pin (PWR_FLAG). Connect this symbol to the net that needs a power output but does not currently have one.