For incoming only, do not configure any
dialer maps or strings on the PRI;
in that case we will not be able to dial.
For outgoing only, you can use dialer
profiles, and under the PRI configure
"dialer pool-member 1 min-link 31", which
will cause all the B-channels to be
reserved for outgoing calls.
You could also place a CLID filter
on the PRI ("isdn caller 1") that will
never be matched; in this was every
incoming call will be cleared.