You can turn it off by removing symbol '0' from the Group Sequence. This turns off also following services: TA, DI, MS, AF. The reason is that PS shares the same group type with these services so cannot be turned off separately. This is given by the RDS standard. Note that the Group sequence cannot be empty (this yields in transmission of groups 0A), there still must be some groups on the RDS output, as defined in the RDS standard.
However I do not recommend that solution. If it is really necessary to encode all RDS groups outside the RDS encoder, you need to synchronize your groups with the RDS stream. Time base for the RDS stream is the pilot tone from stereo encoder. Thus you need to fill the RDS encoder's internal buffer and poll for the buffer status. Also, you should keep the symbol '0' in the group sequence so the RDS encoder would generate the PS internally in the case of external driving failure.
The task is not trivial but still can be realized with the P132 family RDS encoders. It would be nice if you can provide more details about what you're doing.
