Variable question about spaces.

This is an interesting challenge. You could change the (blank) value and replace it by hitting the space bar. The problem here is the users could hit the space bar twice and bypass this.

Have you considered creating a "warning layer" to not use the space bar and then creating the following triggers? Here's the catch, each time they press the space bar, they will get the pop up alert telling them to enter text and not use the space bar. If they just keep closing that alert and use the space bar for the remaining fields, SL will still let them continue.  But, perhaps most people would be tricked into thinking they must not use the space bar and go ahead and complete the fields.

There may be a way to accomplish this with JavaScript but I'll leave that up to someone else in the forums.