Sorry for the length... Technology is easy. People are hard.
First Point - A Question
"If the employee isn't willing to meet your expectations of what is required for the team/business to operate properly, do you really want them on the team in the long-run?" I may be reading a bit too much into your post, but if this individual isn't committed enough to the organization to effectively communicate with the team, I'd strongly lean towards "No." Yes, this means that you are faced with a tough situation (especially for a small firm).
Second Point - My Advice
Creating a policy that in effect allows insufficient communication seems a bit backwards (IMO). It's along the lines of creating a rule such as "It's okay to yell at your client no more than three times a year." (smile) Don't validate the poor behavior (unless you want to see more of it). I've also learned that many employees just naturally push these boundaries. Thus, I'd put even money on the chance that if you set a limit, the employee would just go slightly beyond that limit (because he currently knows he can).
If your employee is a part of a team, which requires a specific level of communication, do not compromise that requirement. Granted, people make mistakes and people can fail to meet expectations, but creating a rule to validate an employees suboptimal behavior is just a bad move.
Third Point - Steps to Follow
I've used the following framework to address employee issues in the past. It's not rocket science, but you'd be amazed at how many of us gloss over different steps.
1. Clearly communicate your expectations to the employee - It's shockingly common for employees to simply not understand what you expect (even when you feel things should be crystal clear). No, this is not done in an email... this is a face-to-face, boss-to-employee discussion. It sounds like you have done this.
2. Discuss the "why" for your expectations - This is when you get to communicate (two way, ideally) about how this expectation is needed for the company to succeed. The employee should be given a real opportunity to share his/her opinion and even make a case to change your mind/expectation. Ideally, you will also be able to better understand the employee's "why" behind his/her action. It should be clear though that, in the end, the decision on how this is handled is your decision. Other than sharing that your employee knows he can get away with it, what do you feel his motivation is on this issue?
3. Set some consequences - Yes, this is the hard part, but there are always consequences to an action. Ideally, the prior steps have established a decent amount of communication so that you can be direct with your employee. The consequence may boil down to, "If you don't meet my expectation, then I will simply not be able to trust you... which means you will not, in the long-run, be a part of this organization." This is a relatively mature and honest statement, but not appropriate for all relationships. Less ideal, but certainly worth considering, are the traditional 'sticks' (a la 'carrot and stick') such as decreased bonus opportunities, more strict reporting requirements, etc.
Fourth Point - A Case From My Own Experience
At a prior employer, I had an employee that had significant issues with showing up on time. Steps #1 and #2 went quite well. We were able to communicate very openly about the situation, and I had great hope that things would improve. They did, but only for a little while (about 2 months). The consequence was that the employee was required to email me upon arriving at work (to document the arrival time). A completely annoying and arguably childish situation, but, hey, that's kind of the point. That consequence motivated the employee pretty significantly (it is a tad embarrassing). We did that for a couple months, with very real improvement. After the requirement was dropped (based on the improved performance), things slowly went back to the original, unacceptable situation. In the long-run, the employee was obviously not destined to play a key role in the organization. One could argue that it would be most efficient to recognize this upfront and simply make the tough decision. That's obviously something that is completely subjective and dependent on the person, organization, situation at hand.
Hope this helps. Best of luck.