There are a number of ways to manage referential integrity.
You could disable the contraint:
http://stackoverflow.com/questions/159038/can-foreign-key-constraints-be-temporarily-disabled-using-t-sql[
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But you wouldn't be able to re-enable it until the data was fixed.
You can configure cascading deletes. This means any record linked through a foreign key would also be deleted.
http://www.mssqltips.com/sqlservertip/2743/using-delete-cascade-option-for-foreign-keys/[
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But this will change database behaviour and allow parent records to be deleted where you may want the presence of a child to create a blocking action.
So in short, no you can't truncate a table without dropping the constraint, but SQL does have mechanisms for helping with referential integrity.