Open manage | Database | Relationships
Find the line linking these two tables and double click it. Clear the option at the bottom of this dialog titled "Delete related records in this table when a record is deleted in the other table."
You may have multiple relationships to check as any two linked table occurrences with the same two data source tables may have this option enabled.
The other work around is to change the value of the field used to link this record to the related records first so that there are no related records, then delete the record.
This option is very useful in many database designs as you often want the related records to be deleted from a child table when you delete their parent record.