Laura, Let me use 'firmed' instead of 'fixed' in my reply.
1. Firmed Planned Orders:
There is an option in the MRP type config called 'Roll forward period'. This option can be set with a -/+ sign to delete firmed planned orders either in past or future respectively.
Any planned order rendered useless but still lying in the system just because of the fimed indicator can be deleted with this setting.
A firmed planned order on a maximum can firm the BOM components underneath and there are no follow-up actions/responsibilities/obligations required for planners, if required.
A planning re-run may sort the issue quickly with out further consultations required with shop floor personnel for instance.
Hence the deletion option is made available for firmed planned orders in SAP standard.
2.Firmed Purchase Requisitions:
Other than manual changes which make the PR firmed, there are other related actions like PurReq release procedure/status which also makes system consider that the PR is firmed and does not allow further changes.
During Planning run system does a simple source determination and during the purchasing source determination there is an extended set of activities that come into picture.
So the message is a lot of follow-up activities are still possible with respect to a PR and hence in standard we do not have some similar option for PR's like the roll-forward period available for planned orders.