'Team Forestress 2' won't delete?
macOS apps like Steam try to keep files you're not supposed to interact with directly (like the resource files needed to run Team Fortress 2) tucked away in the normally-invisible "Library" folder. You can either reinstall Steam, and tell it to uninstall TF2, or you can manually remove the files it left behind. To manually remove it:


Start in the app, Finder
Open the Go menu
Hold option—you'll see a new item appear; Library
Click Library to open it (then release
option)
Open Application Support
Drag the Steam folder to the trash


Note that there may be files you care about, like non-cloud-syncing game saves or other Steam apps, in that Steam folder. Almost everything in Steam can just be re-downloaded later, but in doing the above you naturally assume the risk of damaging or losing anything stored there.

Having typed all that, naturally I just found the Steam Support article saying the same things.