You can add filter before map to remove all bedsAssign items which are not matched with current row.id

{
  tenants.map((row) =>
    bedsAssign
      .filter((u) => row.id !== u.tenant_id)
      .map((u) => (
        <MenuItem key={row.id} value={row.id}>
          {row.fullName}
        </MenuItem>
      ))
  )
}

If you want to break the loop, you can try to use some or find with a proper return for map

{
  tenants.map((row) => {
    const isAssigned = bedsAssign.some((u) => row.id !== u.tenant_id)
        return isAssigned ? (<MenuItem key={row.id} value={row.id}>
          {row.fullName}
        </MenuItem>) : null    
  })
}

You can not break any array methods like forEach, filter, map etc. If you encounter a scenario where you want your loop to break then you should use traditional for loop.

use a filter instead of map for bedsAssign:

{tenants.map((row) =>
                 bedsAssign.filter(
                   (u) =>
                     row.id !== u.tenant_id && (
                       <MenuItem key={row.id} value={row.id}>
                         {row.fullName}
                       </MenuItem>
                     )
                 )
               )}

the filter is going to only fill in the items that are meeting the condition you want.

EDIT: I noticed that you want to break once condition is met, this would work in this case:

{tenants.map((row) =>
                 for(let i of bedsAssign){
                     if(row.id !== i.tenant_id && (
                       <MenuItem key={row.id} value={row.id}>
                         {row.fullName}
                       </MenuItem>
                     )){
                         break
}
}
                 )
               )}