I have a drop down box form field (with values level1,level2) in SharePoint list. How can I lock the drop down box selection in the edit form.aspx whenever the condition is level2?
Out of the box, there is no way to lock a drop-down.
The only solution I see is to attach a custom script to the page that will delete the level1 option as soon as level2 is selected. Then there's no way back.
Note that it only locks the edit form (your question). Users would still be able to use a datasheet view, Web services or other to force the value to level1.
You can do something similar to what you want, if you use an InfoPath form as new/edit page for your list.
Here is an example on how to make a custom InfoPath new/edit form for your list.
See here how to customize list forms with InfoPath
http://sharepoint.microsoft.com/Blogs/GetThePoint/Lists/Posts/Post.aspx?List=8d9e2a99%2Df288%2D47c2%2D916b%2D2f32864f7b82&ID=320 And here
Havent looked into locking dropdowns in InfoPath, but this should get you going so you can experiment with that part of it.
Another option is to create a custom ASP.NET new/edit form
Here is an example on how to do this
as suggested by Mark, i used custom script to disable the dropdown based on condition
var getselectTag = document.getElementsByTagName('select');
if(getselectTag[j].title =='Support Level')
if(getselectTag[j].value =='2nd Level')
getselectTag[j].disabled = true;