This worked for me, using Numbers '09.
In Numbers Preferences, uncheck the box for Automatically detect web and email addresses.
In Numbers:
Widen the cell containing the link so that you can click in the cell without activating the link.
Click in the cell once to select it. Copy.
Press Delete to clear the cell.
Go Edit (menu) > Paste Values.
When finished, go to Preferences and re-check Automatically detect web and email addresses.
Alternate procedure:
In Numbers Preferences, uncheck the box for Automatically detect web and email addresses.
In Numbers:
Widen the cell containing the link so that you can click in the cell without activating the link.
Click in the cell once to select it. Copy.
Set the cell format to Text (This may be done earlier in the process, and may be done for several cells at the same time.)
Press Delete to clear the cell.
Go Edit (menu) > Paste and Match Style.
When finished, go to Preferences and re-check Automatically detect web and email addresses.

The first procedure was used on the top and third cell in the list; the second was used on the bottom cell. All four contained the (fictitious) email address shown, entered in the top cell, then filled down to the others. All four were automatically recognized as email links when entered. Those shown in black type were not recognized as links after the procedures above.
I'd be interested in hearing if a similar technique works with Numbers v3.
Regards,
Barry
EDIT: I see Ian has checked this out with Numbers 3, and posted while I was writing. đ
B