Then you destroy their work area with a hammer when they come back for more help on the same thing. (obviously humor)
It's just like teaching.
Source: I'm the 'excel guy' in my service area. I tell them I'll show you twice and I'll make sure you don't have any questions. Then I'll show you youtube videos. I'm not in the business of 'doing' for anyone.
Being able to share mastery is a key part of being a master at a subject, which is a political and image advantage.
I used my excel ability to learn how to best explain vlookup and got good at it.
I did get taken advantage of early on, but I learnt how to deal with those specific types of users, and not do their work for them.
In contrast to those people, there are others who genuinely respect you for your help, and then use your training to make their life and yours better.
Not to mention, many excel problems are not that hard. If you know what you are doing, it’s an easy win/goodwill for you, and a major problem for someone else solved.
It rapidly recedes from the realm of reasonable considerations once we're talking about someone providing informal Excel support to colleagues as an extracurricular activity.