What would I use to stop cargo ships from destroying my town? 

Any ideas?  This is what I have in the zone now:

/zone undestr true
/zone noupgrade true
/zone pvpgod true
/zone autolights true
/zone ejectsleepers true
/zone nobuild true
/zone nocup true
/zone enter_message "Entering Town no PVP allowed and sleepers will be ejected."
/zone leave_message "Leaving Town you are no longer protected so be on guard."
/zone name Town
/zone noplayerloot true
/zone nostability true
/zone lootself true
/zone nodecay true
/zone nodeploy true
/zone noentitypickup true
/zone nofalldamage true
/zone nohelitargeting true
/zone noremove true
/zone nosignupdates true
/zone nostability true