I have to say - at first read, I don't see that using the office as an ad hoc meeting space is so inappropriate. The company pays for the square footage and people are using it in a productive way.
From a visitor's point of view:
- an office is easier for people meeting you to find
- a visitor may not have a sense of when and how formal meeting rooms are used
- a visitor may have a patterns of needing to take calls and ad hoc meetings that don't gel well with the rest of the schedule for the office - so having a guaranteed free room to dash into makes a great deal of sense
My impression is that if you let the visitor into the building, the visitor has the right to expect to be able to find a place to get his job done, even when it doesn't fit the typical pattern of the office. In many places I've worked, local managers, consultants and visiting execs will do what you describe, because they all need rooms with doors that close.
I wouldn't try to swim upstream in this particular river. I'd aim to angle around the current to avoid the stuff you hate (being distracted while working by a non-sound proofed conversation) while avoiding stuff that isn't really your call unless you happen to be the site manager (ie, where visitors are allowed to set up space).
As ever, the "don't do this, it bugs me" doesn't usually inspire people. People are inspired to change their patterns when they stand to benefit. My strategies would be:
Disable the speaker phone button on the landline phone. The classy way is to replace the speaker phone with a phone that doesn't have speaker phone capabilities. Big offices may have both models lying around - often non-speaker phones are used in cubes, so you may just be able to swap them out.
Failing that, stick a note on the phone and on the inside of the office door that says "room not soundproofed, please be aware during confidential conversations" That sounds more like "hey, you probably don't want to loudly converse about trade secrets in here" and less like "you're bugging me!!!"
Find a better, alternate location for your loud guests. Don't start the conversation with the office manager that is "these folks are annoying, can we lock them out of the spare office?", start with - "what's the best place for these folks to sit where their conversations will remain private?". That may mean that the room gets more soundproofing. :)
Really do relocate something useful in there. Maybe you do a team standup in there every morning. Or you situate the coffee pot in there so folks are in and out all day long. Make the room a team resource and then it's harder to use for contractors. If this doesn't help, 6 weeks after you start, go to the office manager and say "hey, this room serves a useful purpose... is there another good place to put visitors?"