This is not currently supported, though it has been requested before. If it is important to you, you can vote for the issue here:
Incidentally, if you put the user's @-mention name as part of the search (e.g. "@SteveSweales some search terms") it will limit the results to either 1) messages that actually contain the string "@SteveSweales" or messages from that user. It's not the perfect solution, but does provide a measure of flexibility that might help you out for now.
...have mention name FirstnameLastname. Fe.: @MaartenCautreels This certainly helps to know who is being mentioned in a conversation. Imagine your colleague sends the following message...
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