There 3 ways I can think to do what you are wanting to do.
1. If you can get nvault_delete function added
2. Log the person steam ID with const value of gaben and then check that value to set their global. When you want to remove them, set the value to something else. This will add more overhead though.
3. Read the timestamp for a certain entry and prune with that as your window. There is still a slight chance you could remove someone you didn't want to that was gabened at the exact same time. This may be the best option for actually removing the entry from the vault.
Code:
new temp[9]
new delete_time, start_time, end_time
nvault_lookup(vault , arg , temp , 8 , timestamp)
delete_time = timestamp
start_time = delete_time - 1
end_time = delete_time +1
nvault_prune (vault , start , end )
nvault_close(vault )
It's not all of the code, but the idea is there and nice and bunched up for you