While not so kind to the users, as they will only find out after they try to save the record, you can create a field by formula and mark it as being unique. Quick Base will then prohibit the saving of the record .
so for example
If you created a field called [This Client is already being work on by another rep]
if(ToDays(Today() - ToDate([Date modified]))<90, [Related Client])
then that field would either contain the value for related client or would be null.
Th reason for the verbose field name will become apparent when you test this.
You will need to decide what is being enforced as Unique and note that you may not use a lookup or a summary field in the unique formula field.
