All terrorists suck, and they haven't any right to use services like yours to coordinate hurting other people. Nor does anyone have the right to incite violence. But people do have a right to political opinions, and some such might be interpreted as "remotely pro terrorist", without actually being coordination or incitement.
Finding the right lines for these determinations is hard, and I don't envy anyone the task of drawing them. And again, things may be moving fast now, and I understand that if there are real concerns about safety that a broad brush might be the first starting point for you. Even so, I hope that whatever policy you adopt iterates over the fullness of time to something that respects people's rights to express their opinions.