No, the person cannot be compelled to move for sitting in a seat when there are no seat reservations in force.As a hypothetical question, if you found someone else in a seat for which you had a confirmed reservation are you entitled to ask them to move and, if they refused, would the TM have any powers to compel them?
If there are no seat reservations provided, then there is no seat reservations. There is no way to know which seat is 'reserved' so if invisible seat reservations were enforced, it would lead to a game of "musical chairs" at every single station. (Sadly, some XC guards do adopt a policy of enforcing invisible seat reservations, with unfortunate consequences of chaos and blocking of aisles and mass seat swaps at every station!!!)
It is the policy of most TOCs that a lack of reservation labels/displays means the train is effectively unreserved. That's a sensible policy.
In the unlikely event of no seats being available, then the Guard may use his/her discretion to upgrade you to 1st class or alternatively you may be given a compensation. Although a ticket does not guarantee a seat, if you have a reservation for a seat and none is provided it is generally the policy of many of the longer distance TOCs to give some sort of compensation. For example if EC cannot provide you with a seat at all, when you have reserved one, then you will get a 100% refund on a single ticket. See http://www.eastcoast.co.uk/about-us/passengers-charter1/seat-guarantee-scheme/ I think that is very reasonable as you therefore get the travel for free. Other operators may have similar schemes - check with the TOC you are travelling with.
(By the way I merged this with an existing thread as the exact same issue was already being discussed.)