Delete all Stored Procedures at once

If this is a one- time task, just open Object Explorer, expand your database > programmability and highlight the Stored Procedures node. Then turn on Object Explorer Details. On the right you should see your list, and here you can multi-select - so you can sort by name, choose all procedures that start with aspnet_, and delete them all with one keystroke.

If you are doing this repeatedly, then (assuming your procedures are all in the dbo schema):

DECLARE @sql NVARCHAR(MAX) = N'';

SELECT @sql += N'DROP PROCEDURE dbo.'
  + QUOTENAME(name) + ';
' FROM sys.procedures
WHERE name LIKE N'aspnet[_]%'
AND SCHEMA_NAME(schema_id) = N'dbo';

EXEC sp_executesql @sql;