What 0x01AA meant was that when you establish the Foreign Key relationship between the Signup table UserName column and the Pers_D, Edu_D, and Dep_s Username entries you can add a Constraint ON DELETE CASCADE:
SQL Server: Foreign Keys with cascade delete[
^]
What happens then is that when you delete the row from the master table - Signup - the system will automatically remove all teh rows from dependant tables for you:
DELETE FROM Signup WHERE UserName = @UserName
Will do it automatically for you.
And don't use LIKE with DELETE! (Or even UPDATE - keep LIKE for SELECT where possible) it's very dangerous.
Suppose you want to DELETE user "old" and you say:
DELETE FROM Signup WHERE UserName LIKE '%old%'
The user "old" will go - but so will "bold", "oldsmobile" and ... "Stack Holder".
LIKE is dangerous - be very careful where you use it!
I would also suggest that using the UserName as a key field is a bad idea - use an ID value instead and it saves space, saves time, and allow the user to change his name without you having to update all related records.