"I'm sorry, I can't do it this time"

"I'm sorry, I'm not available to help you right now"

"I'm sorry, I'm busy and won't be able to assist you"

It's the 'nice' way of saying NO : )

It may only feel difficult for you at first but you'll get the hang of it. The people who are constantly coming to you for things have made a habit out of it because they know you will oblige. A couple refusals of unreasonable requests and they will get the hint (message) quickly and start to leave you alone or they will search for someone else to take advantage of.