constraint/rule/trigger - insert into table X where not in table Y

Howdy,

I'm trying to think of the best way to handle this situation.

I've got 2 tables, X and Y

Table X has a field foo varchar(20)
Table Y has a field bar varchar(20)

I want to enforce, that if table X.foo = 'dave' then you can't insert (or update) Y.bar = 'dave'

I know this is ideally done in the app, but in the interest of time to market i'm looking into
a simple DB solution.

I know i can do this with a trigger, but was wondering if there was a better way, like using
a constraint or a rule (seems like that would be pretty slick - but i can't wrap my head
around how to make it work)

Thanks

Dave