How to make the item be issued 1 time and it could be thrown away

I apologize for the late reply, I have been experiencing power outages due to a windstorm.

You can issue the item one time by using the first-join or first-world trigger.
The item can already be allowed the ability to be thrown away as long as you do not define the self-drops or death-drops itemflag. The inventory-modify itemflag can also be restrictive via click dropping.

Example item issued once;

  example-item:
    id: DIAMOND
    slot: 0
    triggers: first-join
    enabled-worlds: ALL

Let me know!~

EDIT: Marking as answered due to inactivity, please re-open this issue if you need additional help.