Howto prevent write based on date

I'm a newbie to database admin, but I'm not afraid to try - but this one
has me stumped.

I'm using SQL-Ledger as a front end to postgresql 7.4.2. on (mostly)
Debian Testing, i386.

My fiscal year is over and I would _like_ to prevent any changes to the
data from last year.

I looked/searched in the manual, but I don't even know what to call what
it is that I'm trying to do!

"lock" has another meaning for databases. :)

"Write" seems to bring up lots of user-related stuff.

So, I'm hoping one of you geniuses can tell me where to look, what to
look for, or how to do it. *smile*

Thanks!

Philip, wanabe-admin