Best way to achieve this is to define a FOREIGN KEY
on the table. This is the simplest and most implicit way to do it, some reference on that can be found on MSDN. That means you will not ever be able to a) insert an entry to this table if no corresponding FK exists, and b) delete from the base table if entries with foreign key are here (in your case, delete the user if it already has the settings ). It will look like:
ALTER TABLE NameOfTheTable
ADD CONSTRAINT FK_SomeNameForTheKey FOREIGN KEY (nameOfColFromThisTable)
REFERENCES NameOfTheOtherTable (NameOfTheCOlumnYouAreReferencing);