Within SSRS it's not available as a specific feature, but possibly a work around that gives the appearance of disabled is to set them to Null (or Nothing in the expression).
You have to set the parameter to "Allow Nulls". With this done a parameter has a check box next to it on the report screen. When you put a tick in the checkbox, the textbox for the parameter itself becomes disabled.
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Satya S K J 
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