Can I tell a charity worker to go away in rather more explicit terms?
Surely you are joking with me, why do you want to say that?
I feel hostile already today. I don't want to be bothered. I am too busy right now.
It's not a good idea to yell at others who are just trying to do their job. Sometimes people can't choose where they want to work, so it's best to be as kind and respectful as you can to them to make their days a little better.
The stupid government is wasting my tax money on them.
Charity workers have a job like you and me, and they need to do what they have to. Tax paying has nothing to do with their work, keeping that separate might change your mind on this. It can be frustrating to you that they are always bothering you for money but politely be kind, they have feelings too.