What is the best course of action with a non-Muslim female colleague who expressed her desire to embrace Islam for the purpose of marriage, while there is no desire to marry her? And is rejecting her considered a hindrance from the path of Allah?

You did well by not establishing a relationship with the girl. You must be wary of her and not speak to her except out of necessity and while observing the Sharia guidelines. A Muslim is not permitted to marry except a Muslim woman or a chaste woman of the People of the Book. You are not obliged to marry her. If your intention is to flatter her so she converts to Islam through falsehood or deception, then this is impermissible. Instead, let righteous Muslim women influence her, encouraging her towards Islam and clarifying to her that it is the path to happiness. You can also utilize fatwas that demonstrate Islam is the true religion. If she converts to Islam and her Islam is good, and you are able to marry her, then do so. If she does not wish to, then you bear no blame.