Linksys RT31P2. If I disable the port forwarding

The stock firmware from Linksys does not allow to set up a source address in port forwarding to secure incoming connections (unfortunately).
Some of the alternative firmwares like Tomato and DD-WRT offer this option, but are not compatible with this model.

If you want to restrict access via RPD, there is a program that can help to improve security of RDP called "SecureRDP". This program allows you to restrict incoming RDP connections by IP address, Mac address, and computer name. Take a look here. Their download link is sometimes not working properly, but you can get it from cnet.com as well.
Don't miss the forum to this software! It is very informative and presents many good ideas.

If you have any more questions, just come back here and leave a comment - I'm happy to help.
Down