Summary
I was dissatisfied with my job and accepted an offer elsewhere. Now my boss keeps insisting I stay, and enlists others to try and convince me. How do I deal professionally with this?
Context
A medium-sized IT company with offices both in Eastern Europe (where I work) and in Western Europe. One of their major products is an ERP product that is deployed for customers in 20+ countries.
I have been working here for more than five years in various departments. I have worked with my current boss from the first day here when he was a team leader. This complicates things for me now, but also provides me a great insight about what makes him tick.
After working here for a while, I was moved into a project that is very unappealing for me ("The Bad Place") - it is an ERP product with a messy monolithic architecture, error prone deployments, lots of manual work, almost no automatic testing, data transferring based on files, etc.
There is another project that is much more interesting to me ("The Good Place" - state of the art micro-services based architecture using the latest technologies and design pattern with 80-90% test coverage), but that is currently only used by a single client, and has a smaller team.
I was moved to "The Bad Place" without being asked, and when I complained, I only received vague promises about a transfer. Taking into account the whole context, I tried to find an alternative and I was offered a slightly better paying similar position in another medium-sized company.
The leaving process
I told my manager that I am going to leave and explained to him why I cannot work any longer (very old technology that completely demotivates me). He immediately told his boss, but he did not find any solution.
However, I was later contacted by a fellow manager, and by an ex-colleague (prompted by my boss), who both tried to make me stay. As I accepted the offer from the other company, I told both of them that I will leave the company.
The dilemma
The repeated attempts to make me stay are becoming very annoying for me, and I go to sleep thinking about this. My first reflex is to simply block all subsequent discussions with my boss, however, this feels a very unprofessional thing to do and it will also burn all bridges with a person I used to be professionally close to.
I can also try to accept all subsequent discussions, but it does not feel fair for me to listen to so many promises that seem to change from a day to another. It is also somewhat painful to see that a guy I know for years is trying to trick me.
Question: How do I deal with a manager who keeps insisting I stay with no clear offer after I have already accepted another company offer?