Hi Jodie, I find that when you're doing infills, a good brush through at the start can help remove any loose lashes. If you still need to remove some tatty ones I personally use tweezers. Just make sure you hold the natural lash really firmly with one set of tweezers then use your other set to hold the SP lash at the base. It should ping off quite easily if it's already started to come off.
Use remover for any stubborn ones as you don't want to damage the natural lash as Mimi said. If I need to do this, I lift up the lash in question and pop a really thin strip of micropore tape under it. This protects all other lashes as you don't want to risk getting remover on them. Once off, definitely use primer to make sure the lash is thoroughly cleansed of remover.