Coercion is the use of force or threat of force to make someone do something against their wishes or prevent them doing something they wish to do.

There are basically three means for getting people to do what we want them to do.

Argumentation, where we attempt to convince them with our words.

Incentivization, where we attempt to convince them by providing a benefit or reward.

Coercion, where we use or threaten to use force. 

The first two are perfectly acceptable while the third is not. 

I believe taxation falls into the third category.