eliminate/do not allow blank records (i'm newbiew in access)

[dette]

my problem is about data entry in forms. i have a subtable, say

tblLGUandPO : subtable
SiteEstabID *gets the value from the main table
dtmDateVisited *date each LGU or PO visited
strLGUname *has a lookup
strPOname *has a lookup that depends on the entry in strLGUname

my problem is, everytime i choose a new item from strLGUname, the strPOname leaves a blank record and stores the selected item from strPOname to the next record (the form is in datasheet view) and it looks like this:

SiteEstabID| dtmDateVisited| strLGUname | strPOname |
1 | 04 oct 2004 | LGUname1 | POname1-1
1 | 04 oct 2004 | LGUname1 | POname1-2
2 | 05 oct 2004 | LGUname2 |
2 | 05 oct 2004 | LGUname2 | POname2-1
2 | 05 oct 2004 | LGUname2 | POname2-2
2 | 05 oct 2004 | LGUname2 | POname2-3
3 | 06 oct 2004 | LGUname3 |
3 | 06 oct 2004 | LGUname3 | POname3-1

how can i eliminate the blank record?

thanks in advance!