renting a property is a business arrangement, not a social service. arrange viewings only on terms and times you want. Make sure you pre-arrange well in advance with the agent, set times, and ensure that you are there (I have known people lose items during viewings -if you are there you can keep an eye open). I have, before, proposed terms such as 'one hour, saturday week'. If they want more, ask whether they will reward you for the inconvenience – for example, one week rent free.
LL's should build in void periods (and indeed use the time between viewings to update/clean the property)
That said, do not obstruct unless the LL/agent have been unreasonable themselves…..