Disable the cells after project closures

[Excelworks]

I have a worksheet as follows :

A1

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
30-06-09
31-12-09
30-10-09
30-07-09
Based on date in Col A1; I need to disable the entry in the cells beyond that month. eg 1st date is 30-June; so in that row cells should disable July onwards.

Please give a solution.