What are you really trying to "do", perhaps your taking the wrong approach. Re-directs aren't meant to be added and deleted "temporarily".
To specifically answer your questions, there are 2 issues that get in the way of "temporarily" changing a re-direct.
First, browsers tend to cache 301 redirects pretty intensely. After all 301 means "permanently moved", browsers aren't expecting that information to "temporarily change". Anyway the fact that FF or Chrome or whatever is caching your re-direct has nothing to do with dreamhost.
Second reason is that changing a re-direct is actually a DNS change. Typically TTL at dreamhost is 4 hours, which means that if your local DNS server has recently cached the record it's going to keep using that cached record for 4 hours from the time it was cached, even if it changed on dreamhost DNS. This is just how DNS works, nothing dreamhost specific. You can avoid this portion of the delay by using dreamhost DNS as your computer or routers local DNS, which is fine for testing, but might not be what you want to always use.
You could also avoid the second using a redirect within your .htaccess file, which might be the best way to go if you really need to "temporarily" change or disable them. Changing that type of re-direct is different, in that it doesn't cause a DNS change, the drawback is there is no "button" in the panel, you have to mess with manually editing your file.