Since delete is keyword in C++ that has a different meaning than delete in SQL, sqlpp11 calls the method remove. There is no detailed documentation available yet, but here is an example that might help if you have read about select statements already:
db(remove_from(tab).where(tab.alpha == tab.alpha + 3));
Removing using multiple tables as condition:
test_sqlpp::Users usr;
test_sqlpp::UsersForms usr_forms;
test_sqlpp::Forms form_;
db(remove_from(usr_forms).using_(usr, form_, usr_forms).where(
usr_forms.iduser == usr.id
and usr.username == username
and usr_forms.idform == form_.id
and form_.name == form_name
));