how often do you run MRP?
imagine you have a requirement of 10 from sale order / PIR , a PR is created. you converted that into PO and on the same day subsequently MRP is run. IF the PO is excluded, doesnt it create one more PR which upset the planning?
in your case, probably you can create a program which mark the del complete indicator based on the parameters you decide; schedule this program as job to run at defined intervals