In addition to June7's advice and suggestions,
Its a Multi-User DB within a network, not so elegant I know.
sounds suspiciously like this is a single copy of a non-split Database, accessed by multiple-users on the network; is this the situation? If so, addressing any other problems, until this is rectified, is essentially useless!
Also, exactly what constitutes data entry being 'not allowed?'
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007