Hello Shyaggi,
I see two possible ways to tackle your problem.
Change the query of your dataset to return only the results you require.
or use 'Expressions'
Expressions are REALLY useful in SSRS,
http://technet.microsoft.com/en-us/library/ms157328.aspx
Have a look there for a list of common Expressions and examples.
To enable an expression click the cell you want to change (i'm presuming you have a datagrid to display results on) and in the properties, under value you will see 'Expression'
Opening this will bring up the expression builder where you can build up an expression.
i.e
Add a filter so that IIF Fields!AccountID.Value=x.
You could take this a step further by adding a user parameter which would allow the user of the report to specify the filtering.
Either way you decide to tackle the problem (expressions or dataset SQL) i'd recommend reading up on expression, the majority of SSRS reports will use them in at least one place :)