Why can't he take off the collar?

I think the collar is the equivalent to an Alpha bite.

A Dom forcefully put the collar on Miki and psychologically and cause he's a sub he cant take it off.

Because yaoi magic. ╮(￣▽￣)╭

There is literally no explaination/logical reason as to why it's not possible.
Which makes this a very bothersome story aspect for me (since we saw he caught attention with the collar being shown and the guys wanted to mistreat him immediately - that collar is made of leather, get a knife and cut that shit off is the only logical thing, especially if it's not been given by a partner you currently stay with..).

There are quite a few things that aren't worked out well with this setting. But I suppose it's just to appease to the kinky masses anyway.