They very frequently fall into exactly the same trap. It is - as you say - very difficult to charge a client for rectifying a single spelling error - especially if you made the error in the first place. Let's assume that you didn't, however, and that all these tweaks are in addition to the original brief.
What you do is email the client and say:-
Hi, I just thought I should let you know that we've reached the sign-off date for your site. From now on, any amendments will be chargeable at the rate of X£ per onsite hour, so it will be in your interests to collect any tweaks or amendments together into batches. That way I can make best use of an hour's site work, and you'll get value for your money.
In business it's best to be very upfront about money - don't be afraid of the word, and make sure that the client knows there's a specific cut-off point ("From now on"). That way there will be no sneaking a page of alterations in before the gate closes.
As for the fee - you'll have to decide that for yourself. In the industry I've seen fees ranging from £20 per hour on the server to £200 per hour. I suggest that you keep your own figure towards the bottom of the range, but you must decide.