Expelled from undergrad university for poor academic performance, can I go back there (years later) for Masters? I want to obtain a masters degree at a certain top university (in Canada).

However, I was kicked out of the department due to my bad performance. Repeated attempts at getting back in were unsuccessful.

I have since then fixed my lack of discipline, achieved top grades at a "lesser" university, graduated with a bachelor's degree, and have worked (in a relevant field) for 7 years.

I was wondering how, or if I should even consider applying to my original university.

The reason is purely for my self confidence and, I guess, my ego. I got into that top university with excellent grades, but I was not ready to become an independent adult; computer games dominated my life until my final semester when I finally couldn't hang on anymore.

Graduating with top grades at another university showed that I picked myself up, but I want to prove to my family, the university, and mostly to myself, that I have the abilities to achieve what I could not 7 years ago.

The university is the top university at the field I study, in Canada. US have a couple comparable or better ones, but I cannot afford to go to those...

I know I can apply to another university, but I really want to find a way back in to my old university, and only apply elsewhere if no other options are available...

Any advice is fully appreciated.

Update:
Clarification - I am asking this because that university has my status listed as "may not continue in Faculty"
Keep trying!  I was on academic probation (at a Canadian university :-) ) after my first year of undergrad (2.88 gpa). Thankfully I did not get kicked out. I persevered and now have an MSc and PhD and have been doing medical research for many years. Don't take this as gloating, just trying to say anything is possible :-).

Now, it will depend a lot on the university or department, but if you can convince a faculty member in the department that you want to do research and that you have completely changed your attitude, maybe they will have some sway (at least with the department).  In my field, it is primarily a matter of convincing a faculty member (assuming the grades in the "lesser" university were good).  Again, this will depend on the department.

Alternatively, maybe you can make an appointment with the admissions department and make a case to them why you should be allowed back in to the university.  I suspect many would take your 7 years of experience as a growth experience.

Good luck!