Our next update, v1.5, which should be available in the next few weeks, will offer an easy way to select and delete all items. It will also offer Groups (folders) so you can easily organize your pins and other content by destination, and browse or search within a single Group. It will include a new Edit mode which will allow you to select multiple items in one go, and then move them into a group, or delete them all at once. There will be a Select All button for convenience.
Until then, you may be interested to know that with the current version you can search the My Pins and My Listings tabs by entering a city name. For example, if you type “paris” into the search on My Pins, it will show you all pins located in Paris. On the My Listings tab, you may also search by POI type, for example by typing “Cafe”, etc.