We recently had to delete the SOA schemas, tables and so on manually on one of our installations. It turned out it wasn't as easy as just dropping the tables space used. After having dropped and recreated the tablespace we wanted to use, the Oracle RCU tool still had problems with using the same prefix we had previously used. After some research, by a DBA, it was discovered that the RCU tool apparently creates a table in the System tablespace called system.SCHEMA_VERSION_REGISTRY$ If we look what is present in that table we se that:

select * from system.SCHEMA_VERSION_REGISTRY$;
MDS Metadata Services
SOA MDS
MDS SOA_MDS
11.1.1.7.0 VALID
N

07-OCT-14 02.24.14.455000 PM

ORASDPM
SDP Messaging
SOA ORASDPM
ORASDPM SOA_ORASDPM
11.1.1.7.0 VALID
N

07-OCT-14 02.24.17.731000 PM

SOAINFRA
SOA Infrastructure Services
SOA SOAINFRA
SOAINFRA SOA_SOAINFRA
11.1.1.7.0 VALID
N

07-OCT-14 02.25.07.011000 PM

BAM
BAM Services
SOA BAM
BAM SOA_ORABAM
11.1.1.5.0 VALID
N

07-OCT-14 02.25.36.480000 PM

So apparently this is where RCU keeps track of which prefixes has been used. After deleting the contents by:
delete from system.SCHEMA_VERSION_REGISTRY$;
We could again use the SOA prefix during the RCU installation.