recommended way to downgrade a port to older version?

[joerg van den hoff]

I seem to have an issue with groff 1.23.0 partly not correctly justifying my own man macro formatted 
documents. but at some point in the recent past I seemingly "cleaned up" and lost access to 
previously installed versions in macports so I no longer simply can activate an older version. question:

most frictionless way to get 1.22.4 installed in my macports tree again (I can either remove 1.23.0 
or deactivate it, no matter) and to lock that installation for the time being (so that it is not 
accidentally updated in the next port upgrade outdated run)?

thx
joerg