First we need a test database and test tables

--- Create a test db
create database DUMMYDB;
go
---- create tables in test db
use DUMMYDB;
go
create table t1(id int);
go
create table t2(id int);
go
create table t3(id int);
go

if there is no foreign keys, you can execute one of follow queries (drop, delete or truncate)

use DUMMYDB;
go
exec sp_MSforeachtable "DROP TABLE ? PRINT '? dropped' " --- drop all tables in current db
exec sp_MSforeachtable "DELETE FROM ? PRINT '? deleted' " --- delete all tables in current db
exec sp_MSforeachtable "TRUNCATE TABLE ? PRINT '? truncated' " --- truncate all tables in current db