You should send as soon as possible all the IOTA from that specific address (say ADDRESS9X) to another address of yours from which you never sent (say ADDRESS9Y). If this transaction succeeds in being confirmed by the majority of the network (or by the COO as it is now) you are OK.
But how do you do this with the current wallet software?
Let's assume you are using the current wallet software to spend your IOTAs. Since you have spent IOTAs from ADDRESS9X (which is generated from your seed at a certain index, say index1) it means that you should have no IOTAs at any address generated from the same seed at a lower index.
Now, someone sends n IOTAs to ADDRESS9X.
So, if you know that ADDRESS9Y is generated from that same seed at a greater index (say index2) and that you never used it to send IOTAs, then you should be safe to use the wallet software, login with your seed and send that precise amount (n) of IOTAs to ADDRESS9Y.