I assume you're using bound forms? If so, you have very little say over what the user gets to see. Go unbound for more control (unbound means you have to write your own updates, inserts, delete statement etc as appropriate - it's more work but worth it imo).
However, one solution I can think of would be to hide the offending textbox, creating a second one and make it pul the value from the bound textbox multiplied by -1.. Remember you'll have to write the value back to the original box when it's updated too!
always get the user to put in a minus sign, it just doesn't make sense in my mind to remove it, precisely because the value has a meaning. if for no other reason consistency... everywhere else this value is used its known as -x, if you start allowing propel to use it as x and then add back the - you are making (light) work for yourself. You'd be amazed how quickly somebody can slam in data into a db using the key pad... and the - sign is top right.
however if you must persist then I'd suggest some code behind the controls on lost focus, and before update events.. call a function say called ForceNegative, say
private function ForceNegative(sngvalue as single) as single
I'd also be tempted to put a validation rule in the table design for this column which constrained the value to be negative, or in the range that is appropriate for this item.
the reason for putting the function call in two (or more places), is that events fire in Access at various times.
placing it in the lost focus event, will force the function to fire when the control looses the focus (so it shows the user what value is in the control right now)
placing it in the before validation makes sure the data is correct before committing data to the db
... its possible to change a value in a control and force a before update without going through the on lost focus.
you could change the function to check if its negative already, eg
if ForceNegative= -1*(sngvalue) then
ForceNegative= -1*(abs(sngvalue)) ' convert the number to negative
else 'the value is already negative.. don't do anything