A late 30 day is a big deal, even once, even with a good excuse. By the way, all you can do is put a "consumer statement" into your file to explain the late. Believe it or not, in some cases a consumer statement can cause problems of their own, as this suddenly introduces interpretations by people reading the report. Anyone can see the consumer statement that can get your credit report, so choose your wording wisely. Some creditors view a consumer statement around an unresolved credit problem to mean that the consumer may be "high maintenance" or "troublesome". In any case, it will not change the credit score #.
Score number is a function of length of credit, lack of 'events', percentage of debt to available credit, and the some other interesting tidbits. Therefore, closing old credit cards out may make your score worse. Closing unused credit sources may reduce your available credit and therefor increase the debt to credit ratio, lowering your score. Some folks have determined that an optimum credit score can be achieved by holding exactly three credit cards, two major and one gas. Having department score credit cards appears to lower your credit score. Hard inquiries or excessive soft inquiries may have an effect on your score.
There is also a time factor. Making major changes to your credit may take months or even years to fully take effect and can have a short term negative effect.
Filing a dispute may not be the best course of action. Call the credit entity that is issuing the 30 day late, explain what happened and ask if they can remove that from your credit report. It'll help if you had a good record with them before and have a good record since. Many are hardasses about it and wont change it.
Some tips: http://www.myfico.com/CreditEducatio...YourScore.aspx