How can I remove an item from a player inventory? Anyone have working example? 

I'd recommend taking a look at plugins such as Give, Kits, etc.

This plugin is dope. You type /viewinventory playername and it pulls up his inventory and you can drag and drop stuff into and out of their inventory. Doesn't work with backpacks, though, just the main inventory.

https://umod.org/plugins/inventory-viewer