Hey guys!

Is it possible to disable a user if the date the user is logging in is 1 year from the user_regdate in the users table in the database?

if (user_regdate+1year => user_regdate)
{
// don't login
} else
{
// login
}

I'm making a forum where the user has to buy access before being able to access the forumarea.. and it has to make the check upon registration..

Is it possible?

Yes.
But I'm not sure how paid membership may be related to this feature. It's achieved different way. Since each payment system has its own payment verification methods, the solution is different for each of such case.

To be honest i don't get the way MiniBB is coded.

The payment part is made, the only thing i have to make is the part where the user_regdate is checked uppon login..

You don't have to check user_regdate. Add a new field to miniBB users table, indicating the date of last payment. Add this field's value to bb_cookie.php. When user is logged in, the value from bb_cookie.php will be available in bb_plugins.php. Code the condition in bb_plugins.php to make all forums opened for the logged user, until his payment date doesn't exceed the limit. You can do the same with user_regdate as well.