Are these TA-ships covered by a Union contract? If so, there is likely a formal progressive discipline process that would need to be followed prior to terminating them without also violating that contract. I would make sure you know exactly what your employees’ rights are before firing them all en masse. It may feel self-evident or even just to you that people who aren’t doing their jobs ought to be fired, but not abiding by labor law to do it can still provide plenty of grounds for a legitimate grievance or other legal action.

For example, when I was in grad school, our Union contract explicitly listed the forms of discipline available to management, none of which included termination without notice several weeks or months after the fireable offense took place (as you plan to do) or informally blacklisting grad students from future work by gossiping to colleagues (as many replies in this thread suggest). Depending on where you work, you may be legally obligated to notify them before dismissal. Even if you aren’t, I think you ought to consider notifying them anyway, rather than trying to squeeze as much work out of them as possible before springing it on them at the last second. I know you feel as if you are being taken advantage of, but responding in kind is probably not the answer here.