If you're talking about in display on the report, setting the number field to 1 decimal place should translate to Enterprise effectively.
If, as I suspect you might be, be talking about the actual parameter prompt in Crystal Enterprise's HTML viewer, then there's unfortunately nothing you can do about this. As I recall, there are like 10 decimal places represented even though this is not specified in the report.
I don't believe that this anomaly is echoed in the ActiveX or Java prompts, so you might want to try these.
Naith