your question is very vague
- what is 'it'?. a control called txtYear or similar? a complete record? and in a single form or a continuous form?
- what are you using to determine a prior year? a date field? a field which just contains the year?
- if a date field, does this also contain a time element?
- what does 'locked' mean? you can still view it (depending what 'it' means) but can't change it?
assuming it is a single control containing a year value and in a continuous or single form, use conditional formatting
expression is..... txtYear<year(date())
and set the format to Disabled
if a date field with or without a time element try
expression is..... txtDate<(date()-format(date(),"y")+1
This disables the control so you cannot select it - so it is locked