abizafu,

> Is it possible to abort the transaction and return to calling program
> (i.e. Builder,Delphi,etc...) from a Before Delete Trigger.

not exactly. You may raise an exception which will abort the current
operation and relinquish control to the caller. eventually, control will
reach the client application (unless the exception gets trapped along
the way, f.ex. by a stored procedure which deleted the records causing
the trigger to fire).
Have a look at create exception and exception in your documentation;
it's as simple as

create exception cant_do_it 'You can''t do it';

create trigger ...
as
begin
if not (<can you do it?>) then
exception cant_do_it;
end

HTH
--
____
_/\/ando