Is it possible to "unsource" in bash?

It is typically sufficient to simply re-exec a shell:

$ exec bash

This is not guaranteed to undo anything (sourcing the script may remove files, or execute any arbitrary command), but if your setup scripts are well written you will get a relatively clean environment. You can also try:

$ su - $(whoami)

Note that both of these solutions assume that you are talking about resetting your current shell, and not your terminal as (mis?)stated in the question. If you want to reset the terminal, try

$ reset

No. Sourcing a script executes the commands contained therein. There is no guarantee that the script doesn't do things that can't be undone (like remove files or whatever).

If the script only sets some variables and/or runs some harmless commands, then you can "undo" its action by unsetting the same variables, but even then the script might have replaced variables that already had values before with new ones, and to undo it you'd have to remember what the old values were.

If you source a script that sets some variables for your environment but you want this to be undoable, I suggest you start a new (sub)shell first and source the script in the subshell. Then to reset the environment to what it was before, just exit the subshell.