I haven't added a flag to anyone yet, but can it be removed after adding it like our tag on other users' profiles?
If it can be removed, you might consider removing it.
The user can create a flag. You need to be very careful about its creation, and understand what purpose the flag is intended for.
The flag itself cannot be removed in the future. If the person who created the flag renounces it, i.e. withdraws their support, the flag can still remain if other users supported it.
For the flag to cease to have its power, there must be support in the opposition.
For more details, read:
Trust flags