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Is this residential or commercial?
A typical commercial lease would prohibit assignation or a sublet without landlord's reasonable consent. You can continue to claim rent from the existing tenant but you can serve a notice of irritancy on them telling them to vacate. If they don't then you do have to raise proceedings for removal in the sheriff court.
You would have to lock them out. Assuming you have the ability to secure the site, yes, but you would also have to go through the irritancy procedure to end the actual lease.
The original tenant has right of access until the lease is ended. The current occupier would have no right of access.