First thing m8, its impossible to delete a player unit, which makes things easier for you anyway
Second thing is everytime a player respawns, a new unit is created for the player
So basically what you need to do is keep track of all the units, it doesnt really matter what they are doing, either in a vehicle or whatever, as the main thing i would imagine for you is to get rid of AI created units when a player disconnects
You need to constantly update this list, which would be an array
then simply deletevehicle for each element of the array.
If a player unit, is one of those elements in the array, it will not be deleted
AI units are normally local to the server, therefore its the server that you need to use to delete these units
Hope that helps