You can set a: 1) Global field, 2) Global Variable, or 3) FIeld in a one record table linked to the table you want to link with an X relationship so that a "lock expression" in Manage Security uses it to control the level of access to the table.
See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a description of how to set this up.
You could also use a validation calculation in Manage | database, but then you have to set that for every editable field in the table that appears on at least one layout...
was caused by FM not automatically trimming a field where I had inadvertently pasted a name with a blank space at the end.
With all due respect, that does not sound like an "eccentricity on FileMaker's part" but a user error on yours. I know of no software that would automatically trim such a copied space. But you could use Trim ( Self ) as an auto-enter calculation to trim off leading and trailing spaces. Just be sure to clear the "do not replace existing value..." check box.
The fields can automatically be set to trim the text entered by setting AUTO-ENTER calculated trim(self)