Select the Symbol, Right Mouse Button Freeze
As Gary mentioned, you can freeze the interface using the 'Freeze' command but one word of caution with this. If you need to add new ports you must unfreeze the block, but if you have changed net names for existing ports these existing ports will get updated with these new names as you add new connections. To avoid this you should use the 'Edit Local Symbol' option to add extra ports, again an option on the context menu. When you save the symbol it will be automatically updated in the schematic.
Not the correct place here but a good point:
Hirarchical Symobl is "unfrozen":
When I connect a net to a hirarchical symbol I get a default pin.
Where can I change the default settings for the this default pin ????
Only via the 'edit local symbol' option to open the symbol in the Symbol Editor. By default pins are set to input on the left and top of the symbol and output on the right and bottom. The whisker length is a fixed value hard-coded in the application and can only be changed in the symbol editor.