Selective blocking of "-iobuf" directive in ISE

[wzab]

I'm integrating MIG generated core with big, formerly written and debugged
design.
The problem however is that the old design heavily relies on automatic
generation of I/O buffers, while the MIG generated core requires the
"-iobuf" option to be switched off.
Is it possible to selectively switch off the automatic inferrence of the
I/O buf for paticular signals (those provided by MIG generated core?).
--
TIA,
Wojtek