do you have any documentation stating that you left the firm? its not a big deal,just tell the tax collector you left the firm on sych and such a date and someone else is trading under your account/name. of course see if your old boss will give you documentation first. if not,then you notify the tax collector because thats fraud.
I am not a professional in tax but i would like to guess as follows: one possibility: they still use your account under the username you used. But same username doesnot mean same person, at least in theory it is true. In that case, when they issue the check, that check should be under another name not your name. In this case, you donot need worry about anything. Another possibility: your office manager doesnot report to his or her boss that you have left. In that case, when they issue the check, i am assuming here that your office manager's boss is in charging of issuing check, then the check is still under your name. But since your office manager wants this check then he or she should "find" a way to "transfer" the check from you to him or her or your office manager should find a way just cash that check. In that case, the best way for you, just like Don's suggestion, you should contact head office make them know that you have left.