Welcome to the communities.

Simply speaking you can't unless you get their agreement. They don't have to agree and you may need to find a compromise around timing, buy-out etc. (its one of reasons its often better to reward excellent performance with a non consolidated bonus). If you really cant reach agreement after extensive consultation and debate then you might considering firing and rehiring but it fraught with difficulty and is likely to destroy engagement.

The alternative is to make this role redundant. But I think you may struggle to fit this into a redundancy situation.

In reply to Keith:

Hi Keith,
Thank you for the response was exactly what I thought. With regards to redundancy would that not be workable with the loss of business resulting in less employees needed in that department?

In reply to carol wraight:

If you genuinely have less work and need less employees then yes it is a redundancy situation. It is questionable if you could/would select this employee (who is surely one of your best employees) but thats down to your criteria.

In reply to carol wraight:

As far as redundancies are concerned you will need to identify the type of job for which the need is expected to decrease. If this goes beyond his specific area you will need to identify a larger pool of jobs with people at risk, and the selection criteria you will apply. Ditto for his own department, where other people who are potentially at risk may be present.
If on the other hand the expected reduction in need is extremely specific to the type of work he does, then redundancy may be an option - but you will need to have extremely convincing arguments as to why only his job is at risk... tread very carefully here.

Crossed with Keith’s post