Navigate to:
Settings > Configuration > Units
Please note that only Administrators with settings permission have authorization to administrate units
You can limit which units are used in inventory for maximum stock and maximum consumption.
How to remove units
1. Go to the settings menu 
2. Choose "Units" (If you cannot see Units button, your user do not have permission to edit units)
3. Tick or untick the boxes to the right by each unit.
Tick to use the unit in inventory.
Untick to hide the unit in inventory.
Example:
Here we have only ticked "l" and "kg"
Therefore, you can only choose "l" and "kg" in the inventory.
If you have previously entered units in the inventory, which have now been disabled, this data will not change, but they cannot be re-selected in the future.