You can select the red and make it any color you want including white or transparent. However you said"and text which is obscuring a rubber stamp" Take a look a the original document and see if you can read the entire stamp. 1 of 2 things is possible 1)something in your scanner is creating the red border covering the stamp or 2)the red border is part of the original document and is covering the stamp.
If it is 1, then you need to determine what is creating the border, probably the software thats being used to scan the document if so you need to change its settings to remove the border.
If it is 2, then you will need to firstly remove the border by selecting it in photoshop and giving it the same color as its background. Then you need to select the text and fill it with the same background color. Finally you will need to use the text tool and add back the text that you removed.
Once you are done save the file in a format that can later be re-edited if necessary such as psd or tiff, both of these support layers, transparency, live text and I think tiff supports vector shapes but don't hold me to it, if not psd will.
If you want transparency make sure your image is on a layer that is not locked which by default if it is labeled background layer will be locked. In that case just drag the padlock to the trashcan.
I use multiple ways to setect and mask a particular color:
once is to use the "calculations" command (under "image" which allows you to blend one color channel with another). It work particulary well when the color you want to isolate is contained highly in one of the color channels (Eg: red for a red channel)
when you have your mask ready you'll probably have some tweaking to do, and possibly painting. but normally it proves minimal work. Don't forget that you can use nearly all the filters to alter your mask (minimum, median etc... those are my favorites)
Another way is to use the color range option (in "select") either with the sampled colors (color sampler) or with a specific set of colors (in the drop down menu). For example choose reds, in your example, and you'll have all the reds selected. Once you have a selection, apply it to your images (using a black mask) to have these areas removed. The use of a levels command on the newly made mask will help increase the contrast (I use level but you can use whatever suits you).
Sometimes you can alter the specific color with a hue/sat or selective color adjustment layer (or again, whatever suits you) to make the red more "red" and all of the sudden, the color range works better.
Hope this helps!
Please post a lores or a section of the image in question.