I have a webmethod which has string parameters passed to it. These are required parameters. But in the WSDL minOccurs is set to 0. Is there any way to change the default behaviour?

Class types are always either optional or nullable.

Read here:

http://webservices20.blogspot.com/2008/11/web-services-xml-nullable-and-optional.html

As Yaron said, the answer is no.

However, don't feel bad: it wouldn't matter, even if you could change it. .NET doesn't validate incoming XML against the schema in any case.